I've always wondered what would have become of Johnson, his programs, and America to follow without the "adventurism" in South East Asia. Johnson was exceedingly popular, had won in 1964 in a landslide, and his Great Society was the most sweeping set of social programs since FDR.
That all fell apart with Vietnam. LBJ's popularity plummeted (thus denying him reelection in 1972), economic problems from having to pay for the war forced NASA's budget to be cut and the Great Society to be a bit neutered in the long run and caused trouble in the 1970's and ripples can even be felt today. Disillusion bred lack of faith in government and the military and traditional society. The counter culture's rise would garner a conservative white backlash that would elect Nixon, and then Reagan, and move American politics to the right. And the stress probably took some years off Johnson's life.
So the evolution of things in the past few decades can really be traced back to Johnson and Vietnam. But, what if Johnson had avoided the quagmire?