The USA has under all Presidents since Truman laid the claim that it can interfere in whatever countries it wants to on the grounds of protecting them from any number of a variety of evil ideologies run by people who, while often unpleasant nasty sonsobitches are sometimes greatly exaggerated from what they actually are as opposed to what US propaganda wants them to be. In the course of all this the USA has picked up a reputation for being a ferociously effective military power, once since its War of Independence. Well.......
In reality the USA lost all but two of the big battles in the War of Independence. Under President Washington we had two of our biggest defeats in our military history at the hands of the Natives. Jefferson almost self-destructed the USA by wiping out what the Federalists had built in terms of a professional military and the USA owed its survival to Napoleon more than to anything it actually did from a military viewpoint. The only US general who'd qualify as a capable strategist and tactician by military standards made his mark on a bunch of rebels who were incompetent backstabbing lazy motherfuckers who could teach Starscream lessons in how to efficiently backstab one's superiors. The USA established its modern badass reputation on....the Spanish Empire. Hardly a distinguished opponent. The US Army in WWI began actual fighting in the summer of 1918 when it could have done so a lot sooner had it behaved like a normal ally would.
The US military has developed a long-standing pattern of going into poor backwards small countries, kicking ass, staying for a while, leaving, rinse wash repeat. And frankly defeating the military juggernauts of Haiti, Guatemala, Honduras, and Nicaragua, as well as in more modern times Panama and the Dominican Republic is.......uh.....yeah, the less said about such use of "superpower" military might the better. The USA in WWII did very well on the sea and on the air. On the ground I sincerely doubt strongly that the US Army would have been capable of a sustained operation, much less anything strategic on the scale of the Nazis, let alone the Soviets. The conduct of US generals, MacArthur, Eisenhower, and all the rest indicates a blinkered approach aimed at straightforward attacks using firepower. That is not really military "genius" so much as a technologically upgraded version of Luigi Cadorna War. Then in Korea a bunch of Chinese infantrymen with light rifles and Soviet jets stalemates the superpower army, and in Vietnam a bunch of even less-well armed Vietnamese with no native air power and very limited artillery smashed the hell out of the US Army and Marine Corps when it was incapable of thinking in any level above "here's the enemy, go smash him. Don't ask what the smashing supposed to do, just smash him." And then in the modern US Army it bases its reputation for skilled use of firepower to defeat the overwhelming, invincible military juggernauts of a disgruntled ex-Commie and a regime that had to use mustard gas to save itself from teenagers running over minefields and even then was within a whisker of being smashed by teenagers. running. through. minefields.
The USA has never at any point fought a sustained war against any enemy its equal. It has never in fact had generals that have shown the least hint of ability or concept of facing such enemies. And again, to put it bluntly if defeating Saddam Hussein is all it takes to earn the record of invincible supermen, I'd rather have the Israeli Army fight for the West as they at least have shown abilities to fight enemies with quality to match and superior quantity in straightforward slugfests of the sort the USA has always avoided. So with all this, where the fuck does the US reputation for being military Wunderkinder come from? In my opinion the USA is a military version of Brian Griffin: it can talk the talk but if asked to walk the walk it would find all those million-dollar missiles would be shot off, the enemy's still there, the shit hits the fan thereafter.
Why do you think the USA is considered good at fighting wars?