Compare to the outcome of the Second World War. We DESTROYED a good portion of the European continent in all ways fiscal and industrial and no small part of the Japanese system. We engaged in reconstruction efforts like the Marshall Plan. Something like 12,731,000,000 dollars were spent rebuilding industry, roads and infrastructure throughout Europe using the plan. Compare the portions of Europe that participated and those that 'stood' aside as declared by the Soviets. Most of Western Europe recovered quicker and more fully than their Eastern European peers and the Marshall plan was one of the first steps towards the creation of the EU. It took nearly two decades to finally finish what the plan put in motion.
Compared to what we did in Iraq it was the right method nation building. In Iraq, we spent military lives, did little beyond lining the pockets of some companies with construction work and mimimal efforts at nation building. Here is a secret.. it takes time, money and blood to build a nation. Anyone that thinks we could go in.. kick out Sadam, help the new government hang him, pat them on the back and run out in less than a decade and expect what we left in place to stand alone.. is mistaken.
But then after WW2 there was a massive amount of sympathy in Europe, and readiness to cooperate, trade and do business with the U.S. Okay, tens of thousands of factories, cities, railways, airfields and fields of farmland had been bombed or broken down in fighting, and a fair bit of the bombing had been American - but it didn't make people here in Europe want to be left in the cold bog rather than accept help. Pretty much everybody loved the GIs, the USAF and the USA at that time, even most Germans - very few people felt they had been seriously wronged by the Americans, that the yanks were there to cheat them or kill them, or to grab their belongings. The Marshall plan was magnificent (and yes, a great way to increase U.S. influence and U.S. goodwill) but it wasn't as if many millions of people were going to feel grudge against the U.S. presence.
(Not even mentioning, the triumphant heroes/liberators and the shocked, freezing, gallant, wounded or fleeing Europeans were of the same realm culturally, and felt involved with the other side long before the war. A shared background, while to many of us the first images that come to mind with Iraq, or Afghanistan, are terrorism, chaotic fighting, executions and dictatorship. And oh yes, drugs...)
In Iraq on the other hand - yes, Saddam was removed by U.S. troops, but that's not quite the same thing as Hitler and Mussolini getting ousted in Europe. Yeah, I know some people would contest it and say it was
the same thing, but there wasn't the same feeling that "you're fighting our war with us". And there wasn't much thinking about what to do once the hot phase of the war was over. Plus I'm not sure but I think bombing in Europe in 1943-45 was often less pervasive per square mile than in Iraq during the two U.S.-led wars. Explosives were certainly heavier in 1991 and 2003 than in the forties.
But on the other hand I don't see the US going cold turkey on military involvement in the wider world, any more than the Roman empire could have functioned without a solid army presence and the occasional war against Germanic tribes or Parthians.