The tribalism part is easy: prosperity. Just like a fed snake is calm, so an extremely prosperous society is peaceful.

A simple example: https://en.wikipedia.org/wiki/Automotive_industry#/media/Fil...

In 1950 the US produced 80% of the world's cars. Similar story for appliances and many other products. That wasn't a normal state. So slowly it went down.

So the US is slowly reversing to its mean. Remember, robber barons, the Great Depression, the New Deal.