We joined the war in Europe similarly late and with similar self-interest, but didn't accomplish or lose as much. So why is our heroic intervention in WWII an acceptable source of national mythology and not theirs? I'm actually teaching a very remedial class in 20th century history right now and one of the points that I'm trying to drive home is that trying to find moments where you say "the USSR was acting based on national self-preservation in this case" and "the USSR was acting based on ideology in this case" sort of misses the point when you remember they were figuring out on the fly what it meant to have a modern, bounded nation state based on a fundamentally internationalist ideology.

Past that, I think you're mischaracterizing the Molotov-Ribbentrop Pact. It reads like two major powers who were aware that their objectives for Eastern Europe were going to collide, violently, sooner rather than later and wanted to put it off for as long as possible. It was a disastrous concession to the Nazis, but it wasn't friendship.

It's also worth arguing that maybe you should widen your scope as long as we're arguing about whether there is a natural, active opposition between fascism and communism. World War II wasn't the only grounds for the Soviet Union to act on anti-fascism as a goal - and it wasn't the only time that it did. The USSR sent thousands of people to fight for the Spanish republic in the Spanish Civil War,just as the Nazis sent their support for the Nationalists. We can see that as a proxy war with self-preservation at stake or we can see that as an ideological conflict, but I think the correct answer is that both sides understood it as both of those things. If you want another example, very early in the existence of the Soviet Union, Lenin offered vocal support and advice for the Bavarian Soviet Republic in 1919, before the Bavarian communists were smashed to pieces by the proto-fascist Freikorps - many of which would become literal Nazis as soon as it was available as a political option.

So, I agree that I'm romanticizing their involvement, but don't view that as a misunderstanding of the events in play. Look at photos from the Battle of Berlin. Their involvement *was* romantic - at least as much as war can possibly be - and as one last point I think it's also worth taking a different, bottom-up perspective that includes the huge number of communist partisans that were independent of the USSR who took up arms against fascists in the 30's and 40's. Stalin vs. Hitler isn't the only way to look at this, and there are millions of dead communists who entered this struggle independent of that top-down conflict.