"If you had told me a decade ago that I would be tackling terrorism," writes Tony Blair, "I would have readily understood, but thought you meant Irish Republican terrorism." Good point. Troubling for voters, though. It's rather hard to predict what foreign policy challenges will arise over long stretches of time. From climate change to Iraq to terrorism to genocide in Eastern Europe to the collapse of the Soviet Union and attendant power vacuum in their satellites, it's a fairly surprising world out there, and we don't get to switch leaders every time there's a new shock. Domestic policy is a bit more static a landscape. The problems that arise -- economic downturns, health care inflation, environmental degradation -- are, if not predictable, then somewhat routine, and a candidates general response to such eruptions can be known well in advance.
Conversely,there's really no way of knowing how any of the candidates will respond to foreign provocations and crises we currently can't imagine. In this, Iraq has been something of a distraction, helping to paper over disagreements on a fundamental approach to foreign policy by letting everyone largely agree on the next steps in the primary topic in foreign policy. The response of our putative leaders, however, if some oil-producing African nation collapses into chaos, tossing world economies into flux but proving resistant to any remedy save an uncertain intervention, or if CHina and Taiwan face off, is anybody's guess.
On that note, Barack Obama and Mitt Romney both have articles laying out their foreign policy visions in this month's Foreign Affairs. Seek, and ye shall maybe find.