It wasn't until Vietnam that the U.S. really perfected the art of killing.
Vietnam was one of the biggest clusterfucks in the history of warfare. We employed tactics that were terribly ineffective, sent soldiers into battle with experimental weapons that were not suitable for duty (first version of the M-16 jammed all the time due to lack of corrosion protection on the bolt, carrier, and barrel...and no cleaning kits were issued because they were "self cleaning"), and had to draft unwilling young men to go fight. Although medical technology had advanced from WW2 and helicopters had advanced to the degree where they were a useful offensive asset, we had shit pretty well figured out by WW2.
While we were going through an awkward teen kind of phase during WW1, during WW2 we got pretty fuckin' good at killing people. Some technologies that come to mind:
-Machine guns (fixed and hand-held) with cyclic-rates above 500 rpm.
-Long range bombers with sights accurate enough to hit a building from 25k feet.
-Tanks
-Fighter aircraft with radio equipment that could be called in to support soldiers on the ground.
-Nuclear bombs: when you can kill 100k with a single munition...you're pretty "good" at killing people.
Now, some "America!!!! Fuck Yeah!!!" since I have a great amount of respect for my grandfathers and the other men and women who served our country and the world in WW2 and I want to use one of my favorite quotes....
The brave men and women who fought for American in WW2 were not the "greatest generation" but we were a nation of hard sonsabitches. The nation had been dealing with the Great Depression for the past decade. More importantly, a much greater percentage of the population lived in rural areas/on farms and had experience shooting, hunting, and tracking. Folks had rifles and knew how to use them.
"You cannot invade the mainland United States. There would be a rifle behind each blade of grass..."
Admiral Isoruku Yamamoto
You train an 18 y/o boy farm boy who's been stacking hay bales and shooting since he was 5 how to fight and then put an M1 in his hands and you have a lethal battle system. The Japanese figured we would never leave our shores since we, as a country, were overwhelmingly Isolationist until the bombing of Pearl Harbor. We showed the world that they had greatly underestimated our ability and willingness to bring the fight to their front door.