Here's a short video explaining the proximate causes of the incursion:

Michael Mandelbaum interviewed in the New York Times: "Throughout U.S. history, our nation has oscillated between two broad approaches to foreign policy, Mandelbaum explained in an interview, echoing a key theme in his book: 'One emphasizes power, national interest and security and is associated with Theodore Roosevelt. The other stresses the promotion of American values and is identified with Woodrow Wilson.'

While these two world views were often in competition, that was not always the case. And when a foreign policy challenge came along that was in harmony with both our interests and our values, it hit the sweet spot and could command broad, deep and lasting public support.

'This happened in World War II and the Cold War,' Mandelbaum noted, 'and it appears to be happening again with Ukraine.'"