As far as I can tell, before WWII we had basically the same cultural problems as the rest of the western world (racism, religious fundamentalism, the human rights struggles of the industrial revolution, etc). Then in WWII, we converted our society into a war machine, in order to accomplish something that if not good was at least necessary. When the job was done, we never figured out or even bothered to try to reverse that. We became strictly predatory. Our way of life is to consume others, with our leaders even consuming their own people. At the same time, we've retained this stubborn receptiveness to any claim that whatever we do serves some higher purpose that excuses however ugly those actions are on the surface. As long as some talking head can find any reason to describe another group as bad people, then it's impossible for us to also be bad people...
It's both much more subtle than that, and less cynical, even if the results are the same. To put it simply, the US has a national obsession with wearing the White Hat, and opposing the Black Hat. (A few early western shows, primarily those aimed at kids, color-coded the good and bad characters in that manner, and it is a useful means of examining hte resultant archetypes.) In WWI, the US went "over there" to pull the French and British out of the quagmire. (Talking historical perception here. The American Expiditionary force was decisive, but was little more than fresh muscle. The keys to victory were British innovations in armor and French
blitzkrieg infiltration tactics.) In WWII, the US styled itself the "Great Arsenal of Democracy" and later embarked on "The Great Crusade" against the fascist Axis powers. After the war, Only the US and Soviet Union had any real strength remaining, as much Soviet industry had been relocated to the far east and US industry was completely unharmed by the war. When Stalin began subsuming everything he could reach into a buffer zone of puppet states in order to provide the
rodina with a shield agains the
next German or French invasion. it fell to the US with it's nuclear monopoly and largely intact military to "keep the lights on" in Western Europe. (Again, reality was somewhat more complicated, but we're discussing perceptions here.) This vision of the US as a bulwark was only strengthened by the Korean
War Police Action and was not seriously challenged until the Vietnam era. In more recent times, the US-led Coalition crushed the invading Iraqi army in Sadui Arabia and swept them from Kuwait.
The point is that Americans have come to expect that, whenever American armies go into action, they will be "on the side of the angels." This is, in and of itself, not a bad thing. The problems with it are twofold. First, it's difficult for a country that has set itself up as a defender for a century to realize that a defender
isn't needed. It's not 1945 anymore, and most countries are not only capable of looking out for their own interests, they are equally able to provide aid and comfort to those who lack that ability. Second, when chastised for interference, or opposed by parties whose agendas are stymied by the possibility of intervention, the result is a petulant "but we're the good guys!" Besides the obvious, this attitude lends itself far to easily to a dualistic castle mentality, not helped by blind America-bashing from a very vocal portion of the European population.