Why our generals were more successful in World War II than in Korea, Vietnam

I'd say because there were more experienced British, Russian, French and other allied generals involved too. (Or are we giving the sole credit for WW2 to the USA?)

And second, except perhaps for Vietnam, US strategy has shifted to deliberately create hotspots and unrest in all the major continents to keep other powers bogged down and uniting against the US / west. It's an extension of the old British divide-and-rule policy that America has expanded upon (the British only used to create unrest, the Americans have expanded on that to also use these hotspots to provide fertile, live ground for training new recruits to warfare while also testing new weapons). That's why the Jews were deliberately sent to middle-east, creating conflict in the middle-east, while India was partitioned because the west rightly assessed that India would not be a blind ally of the west against Russia.