The abuse of the Constitution that followed September 11, 2001, was neither surprising nor inevitable. It was not a surprise, because it wasn't the first time in American history—but the sixth, by my count—that fundamental rights had been violated during spasms of fear over national security. It was not inevitable, because prominent voices might have called the country back to its principles. There is no telling whether such appeals would have stood against the tide, but one man's words did make a difference in the emergency command center at FBI headquarters on Pennsylvania Avenue several hours after the attacks.

The voice belonged to James Ziglar, then commissioner of the Immigration and Naturalization Service, who thinks of himself as a Goldwater Republican. "I'm quite a Bill of Rights kind of guy," he says. At the FBI that day, he gathered with other senior officials for what he describes as a discussion about doing "some big sweeps" in "any community where there was a large concentration of Arabs, to basically do a door-to-door."

Ziglar objected: "I was the only guy in the room who raised his hand and said, 'Wait a minute. We've got something called the Fourth Amendment here. We don't do sweeps in this country. We had a revolution over warrantless searches.'"

His reminder brought hostility from some of his colleagues, silence from others. "You've got to understand, this was September 11, and the level of hysteria was huge," Ziglar says. "Once you raise an idea, anybody who says that's a stupid idea is somehow unpatriotic or soft on terrorism." Yet his comment broke the momentum. Afterward, two ranking officials, who had offered him no support during the meeting, told him they were glad he had calmed the situation.

Ziglar could not entirely prevent his agency from mobilizing against Muslims. "What was originally contemplated," he says, "was far more draconian than what happened." Still, what occurred wasn't benign. More than 1,200 U.S. residents were detained, including 762 seized on immigration charges. Many were abused and threatened by guards while being held, sometimes for months, until the FBI and the CIA cleared them for release or deportation. Not a single terrorist was found among them. Then, over Ziglar's objection, thousands of other men from predominantly Muslim countries were summoned to "special registration" at immigration offices, where those without valid visas were summarily jailed and later deported.

Because immigration practices are generally administrative, not judicial, the executive branch alone could turn these policies on and off at will. Indeed, the government eventually abandoned special registration and the sweeps of Muslim neighborhoods because they were unproductive—even counterproductive, given law enforcement's efforts to cultivate cooperation in Muslim communities to report on suspicious activity. Simultaneously, the FBI adopted stealthier methods, including infiltration of congregations at mosques around the country.

By contrast, the actions of Congress since September 11 are proving so durable that even modest attempts at reform are routinely discarded, and expiring provisions are renewed again and again. The result has been to codify some of history's most significant erosions of the Fourth Amendment, which the framers designed to keep the people "secure in their persons, houses, papers, and effects, against unreasonable searches and seizures" by requiring warrants based on probable cause and judicial approval.

Most notably, the Patriot Act of 2001 emasculated an array of privacy statutes that had been enacted in the last quarter of the 20th century. In 2008, Congress further amended one of those laws, the Foreign Intelligence Surveillance Act (FISA) to legalize the extensive surveillance that Bush had clandestinely authorized during the weeks following the attacks. At the time, while legislators and civil-liberties groups were arguing over the Patriot Act's changes to FISA, Bush was evading FISA by ordering the National Security Agency to monitor Internet and phone communications without judicial oversight. Bush's program sparked outrage in Congress when The New York Times reported it in 2005, but in the 2008 FISA Amendments Act, Congress "gave the government even broader authority to intercept international communications" than the president had given himself, according to a joint report by the inspectors general of the CIA, the National Security Agency, the Justice and Defense departments, and the Office of the Director of National Intelligence.

The inspectors general did not credit the program with many successes. While it "had value in some counterterrorism investigations," they found, "it generally played a limited role in the FBI's overall counterterrorism efforts." Most officials interviewed for the report "had difficulty citing specific instances" when the monitoring had contributed to counterterrorism. A senior CIA official told the investigators that the surveillance "was rarely the sole basis for an intelligence success, but that it frequently played a supporting role." Other CIA officials discounted the intelligence as "vague or without context."

Whether effective or not, the government's enhanced powers to probe into private lives flow from the confluence of terrorism and technology. Together they have placed rights at risk by creating a comfortable impression that the problem has a solution, that the amorphous threat of terrorism can be discovered and repelled by computers trolling for clues, selecting suspects, and predicting their behavior. The more secret the methods, the more effective they seem, because they cannot be examined closely enough to judge their virtues and dangers. Reality and imagination play on each other as terrorism is magnified in people's fears, and investigative prowess is enlarged in people's hopes.

Since beliefs are often stronger than facts, recovery from this era seems likely to be slow. The Patriot Act and the invasive measures it permits are taking on the specter of permanence. So a decade after September 11, 2001, it is worth inspecting the act's impact and reflecting on how the damage might be undone.

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IN THE LONG ARC OF AMERICAN history, the pendulum of liberty swings just so far before forces impel the country back toward the center. This capacity for self-correction, the hallmark of a healthy society, works when the conditions are right, not before. Past detours from constitutional principles have ended as the fear has faded, and the deviations now look dishonorable and perplexing.

The first excesses occurred in 1798 under John Adams, when the Alien and Sedition Acts authorized the government to arrest critics and deport foreigners during an undeclared naval war with France. As the perceived threat diminished, the laws aroused sufficient resentment among voters to drive the Federalists out of power.

The Civil War brought suppression of free speech on both sides. In the North, Abraham Lincoln suspended habeas corpus and employed the army to imprison suspected Confederate sympathizers, a practice the Supreme Court declared unconstitutional after the war.

Woodrow Wilson mobilized a campaign of paranoia against opponents of American involvement in World War I, and Congress gave him the tools of political arrest. The Espionage Act, passed in 1917, and the Sedition Act of 1918 were used to prosecute labor leaders, socialists, anarchists, German Americans, and others who criticized government policies. Intolerance for unwelcome speech spread as the Bolsheviks in Russia consolidated power, but in the years after the war, both the law and the culture moderated. The Sedition Act was repealed in 1920, and the suppression of speech provoked a backlash that coursed through the society and prompted universities, for example, to introduce tenure to protect faculty from political dismissal.

The next surge of violations came with World War II. The 1940 Smith Act, which made it a crime to advocate the violent overthrow of the government, was applied to mere membership in communist or fascist organizations. The 1798 Alien Enemies Act was revived to impose travel restrictions and nighttime curfews on 900,000 Japanese, Italians, and Germans, who were barred from owning cameras, guns, and shortwave radios. Internment camps were filled with 120,000 ethnic Japanese, including about 80,000 U.S. citizens whose release was announced by President Franklin D. Roosevelt shortly after he was re-elected in 1944.

From 1798 forward, these anti-constitutional impulses have been driven by fears and fantasies about the enemy within, a rotating roster of imagined sources of subversion that shift according to circumstance. So after World War II, as the Cold War intensified, anxieties switched from Japanese and German citizens to domestic communists. This was the fifth deviation from constitutional principles. Loyalty oaths for government employees, cunning smears by Senator Joseph McCarthy, accusatory questions about political affiliations before the House Un-American Activities Committee, blacklists of actual or suspected communists—all were overt means of enforcing orthodoxy and punishing dissent. The Smith Act endured as an instrument of political prosecution until the Supreme Court reinterpreted it narrowly in 1957 by ruling that the advocacy of violence had to be linked with "concrete action," not "forcible overthrow as an abstract principle." The statute remains in the U.S. Code but is no longer used by prosecutors.

Dissidents were monitored from the late 1940s through the Vietnam War as the CIA, the Internal Revenue Service, the National Security Agency, army intelligence, and the FBI were arrayed covertly against civil-rights and anti-war groups. The FBI's Counterintelligence Program, COINTELPRO, bugged and wiretapped activists, including Martin Luther King Jr., and spread false rumors of infidelity among the families of Black Panther leaders. By the mid-1970s, after the Vietnam War was lost, the country was ready for a reckoning. A committee chaired by Senator Frank Church, Democrat of Idaho, exposed the domestic spying with such documented detail that Congress was prompted to construct high walls between government and the people's private financial and communication records.

The most inventive of these new laws was the Foreign Intelligence Surveillance Act of 1978, which restrained the FBI by regulating intelligence collection inside the United States. FISA established a court of federal judges to decide secretly on applications for clandestine surveillance (and clandestine break-ins, under a 1994 amendment) where the government could show probable cause to believe that the target was an "agent of a foreign power." That was considerably looser than the Fourth Amendment's standard for a traditional search warrant or wiretap order, which can be obtained from a judge only by showing probable cause to believe that someone has committed or is about to commit a crime and by "particularly describing the place to be searched, and the persons or things to be seized." These requirements prevent fishing expeditions of the kind that intelligence gatherers find helpful.

FISA demanded no such specificity or even suspicion of criminal activity. "The purpose" was to gather foreign intelligence, the law stated. The narrow wording was meant to block criminal investigators from using FISA to make end runs around the Fourth Amendment.

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The law worked well for more than 20 years, but after September 11, counterterrorism officials wanted more. Six weeks after the attacks, the Patriot Act changed "the purpose" to "a significant purpose," a slight alteration with a huge result, opening FISA warrants for use in criminal cases when intelligence gathering is ancillary. Also, the target no longer had to be an "agent of a foreign power." Now, the collection "of any tangible things (including books, records, papers, documents, and other items)" could be done for "an investigation to protect against international terrorism or clandestine intelligence activities." This broad catchall allows searches that reach not only suspects but innocent people who might be several times removed from someone in contact with groups defined by the government as supporting terrorism.

These were among a multitude of amendments, to FISA and other statutes, in the Uniting and Strengthening America by Providing Appropriate Tools Required to Intercept and Obstruct Terrorism Act of 2001, a tongue twister designed to produce an inspiring acronym: the USA PATRIOT Act. It was an intricate, book-length collection of abstruse provisions, impossible to comprehend without comparing the new bill and the old laws line by line. Take this key passage, for example: "Sections 104(a)(7)(B) and section 303(a)(7)(B) (50 U.S.C. 1804(a)(7)(B) and 1823(a)(7)(B)) of the Foreign Intelligence Surveillance Act of 1978 are each amended by striking 'the purpose' and inserting 'a significant purpose.'"

Few members of Congress had time to navigate through the legal complexities before their rush to pass it, which they did by wide margins, 357-66 in the House and 98-1 in the Senate, where the lone dissenter was Russ Feingold, Democrat of Wisconsin.

Disagreements over the Patriot Act's surveillance measures have exposed a symmetry between law-enforcement officials and civil-liberties advocates. Both want to prevent the worst-case scenarios. The FBI is now assigned to head off terrorist attacks before they happen, not just solve the crimes afterward. Civil libertarians are trying to head off government acquisition of potent legal weapons before they can be turned against people for their political views.

Therefore, the dangers of having or not having the Patriot Act are cast in hypothetical terms, which require imagination to appreciate. The words "if" and "could" pepper the arguments on both sides, even after the country's ten years of experience with the law. If the surveillance powers had been absent, we are told, we would have been attacked—although few examples are cited. With the surveillance powers present, we are warned, we are losing a key ingredient of liberty—our privacy—although government secrecy masks the extent of the erosion. We do know that in a complex, modern democracy, the state and the individual stand in wary equipoise. Tilting excessively in one direction or the other could bring anarchy or oppression.

But would it? "All men having power ought to be distrusted to a certain degree," James Madison told the Constitutional Convention. We should depend not on officials' goodwill, he was saying, but on a system of restraints. This is what the Patriot Act has damaged, alarming those who believe with Madison that government will do whatever it is allowed to do.

The act makes noncitizens, even those in the country legally for long periods, deportable for association with any group of two people or more involved in "terrorist activity," a term that is broadly defined. As David Cole and James X. Dempsey point out in Terrorism and the Constitution, the law "potentially encompasses every organization that has ever been involved in a civil war or a crime of violence, from a pro-life group that once threatened workers at an abortion clinic, to the African National Congress, the Irish Republican Army, or the Northern Alliance in Afghanistan."

The law also gives the CIA and military intelligence access to unverified information collected through grand juries. In normal criminal procedure, what the grand jury learns—which can be hearsay, innuendo, or rumor—remains secret unless used in a criminal case, where it is confronted by the defendant and tested for accuracy. No such opportunity exists if it is transferred into the files of intelligence agencies.

Investigators of crimes unrelated to terrorism have also gained more latitude. Under previous law, for example, people could be subjected to so-called sneak and peek searches, in which authorities entered a home or office surreptitiously, if a judge found probable cause to believe that giving notice would adversely affect an investigation. Courts required notification within seven days. The Patriot Act reduced "probable" cause to "reasonable" cause and changed the notification period to a "reasonable time." The American Civil Liberties Union translates this as "indefinite." The Justice Department says the delay "could be substantial," possibly as long as the 90 days permitted after the end of a secret wiretap. If the clandestine search is done under FISA, targets never have to be told unless the evidence is used in a criminal case, and even then, the affidavit justifying the warrant can remain classified and unchallenged.

Roving wiretaps, already available for investigations of terrorism and other crimes, were supplemented with a less constrained alternative that could be issued by the secret FISA court. Designed to follow a suspect who uses multiple phones, the wiretap order in normal criminal cases must name the person under surveillance and the phones or computers; under FISA as amended by the Patriot Act, neither the suspect nor the devices need be identified precisely. There is no requirement, as in a criminal wiretap, that monitoring occur only while agents have reason to presume that the target is using that phone or computer. Without these restrictions, authorities may record random conversations by everyone using those phones, including people suspected of nothing.

The Patriot Act also makes orders known as national security letters (NSLs) so easy to issue that they have become one of the FBI's most popular methods of acquiring data when no crime has been committed. The letters are just that—simple letters on FBI stationery that act as administrative subpoenas without meeting the Fourth Amendment's requirements for probable cause and judicial approval. The head of any FBI field office can now issue them to libraries, Internet providers, banks, and other institutions, demanding records that include a person's financial transactions, phone and e-mail contacts, book-borrowing activity, and the like.

Originally, NSLs were discreet peepholes through privacy walls, aimed at gathering intelligence without having to show a judge that criminal evidence was likely to be discovered. They have now become loopholes. No longer must they require "specific and articulable facts giving reason to believe" that the information belongs to "a foreign power or agent of a foreign power." Instead, the letters may demand any information that is merely "relevant to an authorized investigation to protect against international terrorism or clandestine intelligence activities." Relevance is one of the loosest standards in law, but even that was routinely ignored during the Bush administration by FBI agents, who issued NSLs when no investigation was taking place, according to Glenn Fine, the then-inspector general of the Justice Department.

NSLs illustrate how hard it is to measure the Patriot Act's impact, for the implementations of the law's most significant elements are invisible to the public. Even most members of Congress aren't cleared for access to such information. One who is, Senator Ron Wyden, Democrat of Oregon, a member of the Intelligence Committee, has expressed alarm about what he knows but can't reveal. The government has classified certain legal interpretations of the law, he complains, creating "a gap between what the public thinks the law says and what the government secretly claims that it says."

When Fine studied NSLs in 2007, he was unable to draw persuasive conclusions about their effectiveness, even as he quoted FBI officials who hailed them as indispensable. FBI records were in such disarray that the role of each NSL in each investigation was obscure. The inspector general's most specific documentation showed the importance of NSLs in tracing funds from the United States to foreign organizations on the State Department's terrorist list—financial investigations that produced some criminal prosecutions. In addition, officials have explained that by compiling lists of people's e-mail and phone contacts, they can diagram associations that can lead to far-flung actors in what might turn out to be dangerous plots. Whatever unseen successes this mapping effort may have achieved, its invisible costs can be reckoned in names of innocents and their personal contacts being added to government databases. NSLs are running at a rate of about 50,000 a year, some 24,000 of which in 2010 were aimed at "United States persons," both citizens and legal immigrants.

A gag order accompanying each NSL enforces the letters' secrecy: The order warns the recipient against saying anything about it to anyone except an attorney. In the original law, the gag never came off and could not be contested. After an online court document accidentally identified a consortium of Connecticut public libraries as a recipient, Congress made a slight change, allowing an annual challenge to the gag in court but requiring the judge to defer to the government's claim that secrecy was needed. In 2008, the Second Circuit Court of Appeals ruled this a violation of the First Amendment and shifted the burden to the government to show why disclosure would pose a risk to personal safety, national security, diplomatic relations, or an ongoing investigation. The Obama administration chose not to appeal this decision, so it stands.

Only three recipients of the hundreds of thousands of national security letters have challenged them—the library group, an online archive, and a small Internet provider—and all have won in court or forced a retreat by the government. This is one way to roll back the onerous intrusions: Go to federal court for a constitutional ruling and bring to light the abuse. The Obama administration, though, is making such push-back more difficult by continuing the Bush administration's argument in court that parties who challenge surreptitious surveillance have no standing because they can't demonstrate that they have been under surveillance.

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HOW CAN THIS SIXTH PERIOD OF constitutional deviation be brought to a close? If past episodes in American history are a guide, several developments may have to occur. First, the objective threat must end and with it, the subjective fear. This doesn't mean that the perceived danger has to fall to zero, just to a level of comfortable risk. Having an open society entails some risk. Perfect security, after all, is an aspiration of the police state.

Since the threat of terrorism will not end with a signing ceremony, perhaps it will fade away as the causes of outrage evolve and the targets of violence shift. Terrorism may feel new and everlasting to Americans, but it runs in cycles elsewhere. We may eventually look around and say to one another, "Have you noticed? It doesn't seem to be happening much anymore."

Second, the erosions of constitutional rights have to be clear and obvious enough to mobilize the public. Violations committed in secret are hard to get mad about, especially when you're scanned, frisked, and searched whenever you get on a plane. We mutter complaints, then go along because we are persuaded that we have to relinquish privacy and dignity for security. This sets us up to accept other intimate intrusions. Once people let their bodies be probed and patted and rendered naked by the TSA, they have trouble generating outrage when their bank accounts, e-mails, and phone records are examined by the NSA—especially since they don't know it's happening. Then, too, we all voluntarily disperse personal information through the Internet. Boundaries between the private and the public are vanishing, both in constitutional law and in cultural habits.

It might take egregious abuses to trigger enough indignation to make change, and the violations will probably have to be directed against innocent white Americans who don't have Arab names or foreign accents. A backlash could develop if law enforcement targets citizens for their political views or associations by accusing them of providing groups on the terrorist list with "material support," which the law defines broadly to include "training, expert advice or assistance." This can mean teaching nonviolence or democratic principles, the Supreme Court ruled in 2010.

Finally, even if most Americans someday agree that the era of terrorism has ended, and even if they become sufficiently troubled by the rights violations, the country may still need organized truth-telling akin to the Church Committee. Once the fear has abated, once the public concern is animated, then a truth commission can lay out our government's hidden abuses—not just torture, which has largely been exposed, but the scope of surveillance and its effects on ordinary lives. President Obama might have established such a commission early in his tenure. But he demurred. "We need to look forward as opposed to looking backwards," he said. The president had it wrong. Several other countries with much more painful histories—South Africa, Rwanda, and Cambodia among them—have learned how hard it is to go forward without looking back to expose and therefore heal. We cannot correct until we know what has to be corrected. The excesses of the Patriot Act and other post-September 11 measures cannot be undone until their full results become visible.