After Iraq and other debacles, many Americans are exhausted by the global leadership role. Many have lost faith in the nation’s leadership class. They say correctly that we have a lot of problems here at home.

But a big part of the shift is caused by the fact that many Americans have lost faith in human nature and human possibility.

Even after the horrors of World War II, most Americans said that people can be trusted. Building a community of nations seemed like a doable proposition because most people are basically good. Even today, people who express high social trust are much more likely to see America as an indispensable nation, much more likely to believe American values are universal values and much more likely to support the policies that preserve the liberal world order.

But social trust has collapsed over the decades, especially among the young. Distrustful, alienated people don’t want to get involved in the strange, hostile, outside world.