• Chemical and biological weapons. First, whether or not Saddam Hussein uses them; and then, whether or not large caches of them are discovered. Either one could bolster the perception that Hussein is as dangerous as the US has claimed.
• Iraq's oil wealth. In the short term, whether or not Hussein torches oil wells, which would fuel his negative image in the world; and longer term, how Iraq's vast oil wealth is administered - in other words, how the antiwar argument that this is a war about oil stands up.
• The length of the war. A short war would bolster support for the American intervention. But some critics of the Bush doctrine of preventative war fear that "success" in Iraq would encourage the US to resort more quickly to the use of force in other conflicts.
• The Iraqi population's response to an American invasion.
"No one factor will determine how this war is viewed and what its long-term impact will be. It will be a cumulative effect from many things happening at once," says Patrick Clawson, deputy director of the Washington Institute for Near East Policy.
"If the war doesn't go well, casualties are high, and the US doesn't come up with [the] large stores of weapons of mass destruction it has said are there, perceptions [will] turn against the US. But if the war ends quickly, with WMDs found and a connection to terrorist organizations on top of that, and the Iraqi people show enthusiasm for working with the Americans on building a democracy, then the cumulative impact [will be] very different."
One challenge for the US is that negative opinion around much of the globe - and the fact the US is seen as having lost the battle of persuasion at the United Nations - means it starts with a deep deficit to make up, experts say.
"The Bush administration has done such a terrible job of explaining why this has to be done, they've flitted around from one reason to another, [so] it's going to take a while to chip away the skepticism no matter what happens," says Richard Stoll, a foreign-policy expert at Rice University in Houston.