I think it was the Finnish as well as Spanish< civil war where the winners executed hundreds of leftists/p>
In Spain, you may be thinking about the Derutti column and the anarchists. As the Communists took over areas, the Russian Communists who are misnamed the “Republican Army” executed hundreds of anarchists, who had been the “useful idiots” who had helped the Communists take over by carrying out bombings, assassinations and general disruption for years before the outbreak of the Spanish Civil War.
When Franco won, he sorted out people who had actually been Communist leaders or who had worked in league with the Russians, and while many of that group were executed, it wasn’t a blood bath and most of the people who had fought or worked on the “Republican” side went to jail, coming out as things settled down and the Communists were not likely to be able to regroup. It was no fun, certainly, but on the other hand, he did not drag the entire left out and shoot them.
There actually had been some starry-eyed “Republican” leftists who were not Communists, but the real engine behind the left in the war and beyond were the native Communists and the Russians, who completely controlled the “Republican” forces and government in many places.