> The Cold War undoubtedly helped, though my impression (I am not American) is that there was a greater sense of social cohesion back then (that did not include minorities).

Not to be dismissive, but... no. You're talking about the time of the Civil Rights era and the Vietnam war. When you hear Americans talk about how utterly divided these times are, keep in mind that, on the whole, we have no sense of history and the attention span of a gnat.