One enormous divide--far wider than it is today--was between North and South. Between 1865 and 1940, when more than 30 million foreign immigrants moved to the North, only about one million Southern whites and one million Southern blacks followed suit, despite the fact that wages were more than twice as high in the North as in the South.
The book under review is A Nation Forged in War: How World War II Taught Americans How to Get Along. Evidently, it makes a case that serving together in wartime helped make America better integrated as a society. Much as a libertarian might suspect that this book comes from a conservative point of view, Barone writes that "Readers will come across some irritating liberal pieties," which suggests otherwise.
I think it is possible that the war helped the economy to thrive in the late 1940's and 1950's, by helping to break down the cultural barriers to which Barone refers. My late father told me that without the war, he believed that few of his classmates from Soldan High School in St. Louis in the 1930's would have moved more than a few miles from home. As it turned out, they fanned all over the country.
My guess is that the post-war social network had dramatically more complex connections than the pre-war social network. If the Great Depression required a Great Recalculation, then creating a more complex social network could help new fusions of people to form enterprises.