The American Army has become the killing machine that it is (and yes, killing our enemies is a good thing) because of many factors that have taken generations to complete.
Strategypage goes through them in some detail here.
I'd add only one: unit cohesion. Strategypage knows this well so I don't know why this wasn't mentioned separately. It may be assumed to be included in the general training category but I think it deserves a distinct discussion.
Rotating units rather than soldiers, and using stop-loss to keep critical personnel in their deploying units, have made our combat units more deadly and more resilient when few would have predicted in 2003 how well our Army would hold up despite the strains imposed by five years of war in Iraq and another year and a half fighting in Afghanistan.
As you see complaints about how long it is taking to create a competent Iraqi army that must fight while it is built, remember that a great army is made over a generation by many different strands of varying durations that must be pulled together. And only some of the strands are obvious without the rigorous scrutiny that combat provides.
We've built that Army (and Marine Corps), and we can see the results in action. And we don't see the casualties that an inferior army would have generated to get the same battlefield results.
I hope our Navy and Air Force have the same strands in place, but without combat in their core areas of naval and air supremacy, we can only guess with little hard information to guide us.