Congress is now debating—informally until they return to session on Monday, formally thereafter—whether we should take military action against the Syrian government. But the Obama administration has made clear its belief that it doesn't actually need congressional approval for the strikes it plans to undertake. Are they right? Herewith, a brief explainer on presidents, Congress, and war powers:
Doesn't the Constitution give Congress this power?
The Constitution gives Congress the sole power to declare war, but it also says that the president is commander-in-chief of the armed forces. So the way presidents have usually responded is simply by not bothering to ask for a declaration of war when they want to begin a military undertaking. In fact, the last time Congress declared war was in 1942, when it did so against Romania, or as it was known then, Rumania. There were six separate war declarations in World War II, one for each country in the Axis (see here for more detail on our 11 war declarations; h/t Garance Franke-Ruta). Many of our biggest wars, including Korea, Vietnam, and Iraq, had no formal declaration of war.
Doesn't the War Powers Act require the president to get congressional approval for military action?
Not to be annoyingly pedantic, but what you're thinking of is actually the War Powers Resolution, which was passed in 1973 (there were two War Powers Acts passed during World War II, but those are different, though many people refer to the War Powers Resolution as the War Powers Act, which gets a little confusing). The War Powers Resolution says "The constitutional powers of the President as Commander-in-Chief to introduce United States Armed Forces into hostilities, or into situations where imminent involvement in hostilities is clearly indicated by the circumstances, are exercised only pursuant to (1) a declaration of war, (2) specific statutory authorization, or (3) a national emergency created by attack upon the United States, its territories or possessions, or its armed forces." In other words, the president can respond with force in the case of a direct attack, but in any case short of that, he needs to come before Congress and get permission. The Resolution also requires the president to make reports to Congress when he does take military action.
President Nixon vetoed the WPR, but Congress overrode his veto. Nevertheless, every president since has said he thinks the WPR is unconstitutional and won't be bound by it. The Supreme Court could resolve that question, but the Resolution has never been tested in court.
Don't presidents act as though the War Powers Resolution was in force when it really matters?
Sometimes. It's true that for a pleasant little bombing campaign, they usually don't bother. And in many cases, they'll follow the part of the WPR that says they have to make reports to Congress once the military adventure has begun, but not the part that says they need congressional approval beforehand. Even some major invasions have been undertaken without Congress' say-so. Ronald Reagan invaded Grenada, George H.W. Bush invaded Panama, Bill Clinton waged extended bombing campaigns in Bosnia and Kosovo and sent 20,000 troops to Haiti, Barack Obama intervened on behalf of Libyan rebels—all without a congressional resolution.
Just as there have been 11 declarations of war, on 11 other occasions, dating all the way back to 1798 and most recently before the Iraq War, a president has asked Congress to give him authority to use military force. And all 11 times, Congress has said yes. Syria could, of course, be the exception.