As we mark four years since Sept. 11, 2001, one way to organize a review of what has happened in American foreign policy since that terrible day is with a question: To what extent has that policy flowed from the wellspring of American politics and culture, and to what extent has it flowed from the particularities of this president and this administration?

It is tempting to see continuity with the American character and foreign policy tradition in the Bush administration's response to 9/11, and many have done so. We have tended toward the forcefully unilateral when we have felt ourselves under duress; and we have spoken in highly idealistic cadences in such times, as well. Nevertheless, neither American political culture nor any underlying domestic pressures or constraints have determined the key decisions in American foreign policy since Sept. 11.

In the immediate aftermath of the 9/11 attacks, Americans would have allowed President Bush to lead them in any of several directions, and the nation was prepared to accept substantial risks and sacrifices. The Bush administration asked for no sacrifices from the average American, but after the quick fall of the Taliban it rolled the dice in a big way by moving to solve a longstanding problem only tangentially related to the threat from Al Qaeda - Iraq. In the process, it squandered the overwhelming public mandate it had received after Sept. 11. At the same time, it alienated most of its close allies, many of whom have since engaged in "soft balancing" against American influence, and stirred up anti-Americanism in the Middle East.

The Bush administration could instead have chosen to create a true alliance of democracies to fight the illiberal currents coming out of the Middle East. It could also have tightened economic sanctions and secured the return of arms inspectors to Iraq without going to war. It could have made a go at a new international regime to battle proliferation. All of these paths would have been in keeping with American foreign policy traditions. But Mr. Bush and his administration freely chose to do otherwise.

The administration's policy choices have not been restrained by domestic political concerns any more than by American foreign policy culture. Much has been made of the emergence of "red state" America, which supposedly constitutes the political base for President Bush's unilateralist foreign policy, and of the increased number of conservative Christians who supposedly shape the president's international agenda. But the extent and significance of these phenomena have been much exaggerated.

So much attention has been paid to these false determinants of administration policy that a different political dynamic has been underappreciated. Within the Republican Party, the Bush administration got support for the Iraq war from the neoconservatives (who lack a political base of their own but who provide considerable intellectual firepower) and from what Walter Russell Mead calls "Jacksonian America" - American nationalists whose instincts lead them toward a pugnacious isolationism.

Happenstance then magnified this unlikely alliance. Failure to find weapons of mass destruction in Iraq and the inability to prove relevant connections between Saddam Hussein and Al Qaeda left the president, by the time of his second inaugural address, justifying the war exclusively in neoconservative terms: that is, as part of an idealistic policy of political transformation of the broader Middle East. The president's Jacksonian base, which provides the bulk of the troops serving and dying in Iraq, has no natural affinity for such a policy but would not abandon the commander in chief in the middle of a war, particularly if there is clear hope of success.

This war coalition is fragile, however, and vulnerable to mishap. If Jacksonians begin to perceive the war as unwinnable or a failure, there will be little future support for an expansive foreign policy that focuses on promoting democracy. That in turn could drive the 2008 Republican presidential primaries in ways likely to affect the future of American foreign policy as a whole.

Are we failing in Iraq? That's still unclear. The United States can control the situation militarily as long as it chooses to remain there in force, but our willingness to maintain the personnel levels necessary to stay the course is limited. The all-volunteer Army was never intended to fight a prolonged insurgency, and both the Army and Marine Corps face manpower and morale problems. While public support for staying in Iraq remains stable, powerful operational reasons are likely to drive the administration to lower force levels within the next year.

With the failure to secure Sunni support for the constitution and splits within the Shiite community, it seems increasingly unlikely that a strong and cohesive Iraqi government will be in place anytime soon. Indeed, the problem now will be to prevent Iraq's constituent groups from looking to their own militias rather than to the government for protection. If the United States withdraws prematurely, Iraq will slide into greater chaos. That would set off a chain of unfortunate events that will further damage American credibility around the world and ensure that the United States remains preoccupied with the Middle East to the detriment of other important regions - Asia, for example - for years to come.

We do not know what outcome we will face in Iraq. We do know that four years after 9/11, our whole foreign policy seems destined to rise or fall on the outcome of a war only marginally related to the source of what befell us on that day. There was nothing inevitable about this. There is everything to be regretted about it.