America and its enemies

Why is it that historically when the U.S. has waged war against enemies perceived as conservative or "rightist" (the British during the Revolution, the Confederacy, Spain, Imperial Germany, Nazi Germany, Imperial Japan) it does so with utter ruthlessness, settling for nothing less than total victory, but when the U.S. wages war against enemies either perceived as "progressive" or favored by (some) leftists, as Islam curiously often is (North Korea, North Vietnam, Communism generally, Iraq, ISIS) it does so half-heartedly, ineffectively, and inconclusively? Perhaps the inherently revolutionary and progressive origins of the USA (logically celebrated by American liberals, and illogically ignored by most American conservatives) are impossible to overcome, even for well-meaning Americans who would like to.

(Note: this observation, including most of the specific examples, is not original to me, but I forget where I first encountered it.