> One of the most disappointing things in American politics in the last ten to fifteen years has been watching the left abandon their antiwar fervor

You maybe mixing up anti-imperialism with antiwar sentiment. In the past, the 2 were hand-in-hand due to the geopolitics of the day, but its not a given - depending on the circumstances.

It's interesting how the American right is also taking up an anti-war stance, but maintain the pro-imperialist aspect[1]

1. With Russia's invasion of Ukraine, and the "peace with North Korea at all costs" under Trump