You may not be interested in war but war is interested in you. When it comes to the 2003 US-led invasion of Iraq, Leon Trotsky’s line is apt. Much like the role Vietnam played in American politics long after US choppers had flown Saigon, the Iraq syndrome stalks foreign policy debate. The latest victim is Jeb Bush, who last week gave four different answers to whether he would have emulated his brother had he known then what he knows now. He arrived at the correct one — “no” — three responses too late. As a display of political constipation, it was awkward to watch. Nor is the question likely to go away. Unlike Vietnam, which did not pose a direct threat to the US, Iraq remains a live challenge.

There are two important similarities with Vietnam. Iraq is a mistake that will affect US politics for a generation. George HW Bush proclaimed that America’s swift victory in the 1991 Gulf war had exorcised the Vietnam syndrome. Routing Iraq’s army in a 100-hour desert operation purged fears of US soldiers becoming bogged down in another nation’s jungle. In retrospect we know the first Gulf war gave birth to the second. The fact that Bush senior stopped short of toppling Saddam Hussein sowed the narrative for Bush junior to finish that job after 9/11. George W Bush’s invasion of Iraq ran into the quicksand of urban guerrilla warfare. The terrain may have differed but the lesson was similar to Vietnam’s. It is hard to win a war when your opponent cares more about the outcome than you.

Second, Iraq has become a litmus test of a candidate’s world view. Vietnam also roiled both US parties. It converted the Democrats from the muscular internationalism of Harry Truman into a virtual citadel of peaceniks. For cycle after cycle Democratic candidates went through contortions to appear dovish to their rank and file without appearing soft on national security. The absurdly comical nadir arrived when Michael Dukakis, the liberal Democratic presidential candidate from Massachusetts, posed as a tank commander at a 1988 campaign photo op. But the legacy of Vietnam also affected hawkish Republicans. Even Ronald Reagan shied away from direct military engagements except in cases — notably Grenada in 1983 — where easy victory was assured.