Yet none of the Bush administration’s war architects have been called to account for their mistakes, and even now, many are invited to speak on policy issues as if they were not responsible for one of the worst strategic blunders in American foreign policy. In a video posted recently by the conservative American Enterprise Institute, Mr. Wolfowitz said he still believed the war was the right thing to do. Will he and his partners ever have the humility to admit that it was wrong to prosecute this war?

President Obama opposed the Iraq war from the start and has been single-minded about ending it, withdrawing the last combat troops in 2011. American influence in Iraq has greatly declined since then and Mr. Obama’s attention, like that of most Americans, has shifted to other priorities. Iraqis are responsible for their own future. But the country is a front line in the conflict between moderate Islam and Al Qaeda, not to mention its role as an oil producer. It requires more sustained American involvement than we have recently seen.

Iraq is a reminder of the need for political leaders to ask the right questions before allowing military action and to listen honestly rather than acting on ideological or political impulses. Mr. Bush led the war, but Democrats as well as Republicans in Congress endorsed it. Iraq also shows the limits of America’s influence in regions where sectarian enmity remains strong and where democracy has no real history.

That experience is informing American policy judgments more generally. It has affected decisions about Syria, where President Obama has been right to move cautiously. For a long time the Syrian opposition was divided, and it was hard to know which group, if any, deserved help. It also made sense not to rush into another costly war in another Arab country that could fuel new anti-American animosities and embroil the United States for another decade.

But with the Syrian conflict in its third year, the fighting has already spilled over the borders, destabilizing its neighbors, even as Al Qaeda-affiliated rebels play a bigger role. The reasons for opposing direct American involvement in Syria remain strong, but the United States needs to calibrate its policies continually and should not allow the Iraq experience to paralyze its response to different circumstances.