German commanders not only decided to fight on a multiple front war, they chose to fight a war of attrition against the Soviet Union who had more than double Germany's population. German commanders launched idiotic offensives like citadel and autumn mist. They took a war that they could have won and completely destroyed their hopes for victory. It doesn't matter if Hitler was running everything, they still went along with it.
Britain had Montgomery, who was not only too cautious, but epic fail operations like market garden were his idea. Montgomery's biggest achievement was defeating the Afrika corp in North Africa, which only happened because the Afrika corp was a rag-tag, under-supplied, assistance force that was pushing the British across North Afrika until they ran out of resources because the Eastern front was gettin 95% priority.
Japanese generals had a pretty decent go for the first six months of the war when they were fighting peasant Chinese soldiers and wiping out under-prepared allied forces. Once the war turned conventional, though, and Japanese generals had to fight a prolonged war, banzai tactics and kamikaze waves couldn't hold up to America's legitimate combat doctrine and technology.
American generals proved they were a higher tier right-off-the-bat by pulling out of the philippines once the Japanese initially attacked because they realized that that area could NOT be defended. McArthur's island hopping campaign of by-passing Japanese strongholds and taking only the necessary ones proved to be the decisive blow to Japan. American Generals devised Operation cobra, which utilized the battle of caen(which was going nowhere) as a distraction to break through German lines in normandy. General Patton, America's greatest general, started an American "Blitzkrieg" that tore through German lines all the way to Germany and left other allied armies in the dust. The battle of the bulge was foreseen by General Eisenhower, who then rushed reinforcements to the area and stopped the German offensive. All these reasons add up to why America had the greatest generals of World War 2.
Sorry for bad paragraphs!