A lot of people flipped their opinions on war when their country was attacked in WW II. The Almanac Singers flipped when the USSR was attacked. Before that happened the Almanac Singers recorded their debut album, Songs for John Doe. Here's the full song list. I've listened to it all. It's quite educational and filled with savage attacks on Roosevelt from the left.
Ballad of October 16th
'C' for Conscription
Plow Under (The only one I couldn't find on YouTube)
The Strange Death of John Doe
With the attack on the USSR, the group flipped their politics and wrote pro-war songs. It's hard to find the album as most copies were destroyed when it became politically inconvenient.
The Almanac Singers flip due to an attack on a foreign nation instead of an attack on the US did not sit well and the group soon disbanded. An honest pacifist was fine in pre WW II America but politicking in song to the Soviet's tune didn't sit well with most.