He didn't defeat the U.S.. After years of having the war run by Kennedy's (and then Johnson's) "Best and Brightest" Ivy League braintrust, the spoiled baby boomers, in conjunction with the traitorous news media and craven Democrat politicians, created an unstoppable political force which demanded that we retreat. If Giap had to confront a U.S. force commanded by military leaders, he would have been defeated handily.
His TET Offensive was a humiliating defeat for him, but Walter Crankcase portrayed it as a glorious victory.