American intervention has not been automatic; WashingtonClinton did nothing much to halt the slaughter in Rwanda and Burundi, for example. It has often been late, as in trying to stay out of Bosnia until 1995 (Clinton again). It has occasionally come to grief, as in Somalia in 1993(Another Clinton debacle). More often, though, it has succeeded, as in Grenada (1983)(Reagan), Panama (1989)(Bush I) and Kuwait (1991)(Bush I again).
I'll also add Afghanistan (Bush II) and Iraq (Bush II again).