First of all framing it as “when” Russia invades implies it is some inevitability when it is one of the least likely scenarios imaginable.

Secondly, it would not have anything to do with anti-imperialism to fight on behalf of reactionary, imperialist Europe. Even if you saw Russia as being equally as bad as Europe, the correct position in that case would be to adopt Lenin’s revolutionary defeatism.

But Russia isn’t equally bad. They are on the side of virtually all anti-imperialist states in the world right now. Europe is all-in on imperialism and Nazism. An “invasion” of Europe by Russia would be a liberation from the fascist EU and imperialist NATO. I would rather sabotage my own country from the inside and try to join the Russian forces first chance i get.

As for the US attacking Europe, anyone who thinks that Europe would do anything other than roll over and lick the master’s boots is deluding themselves. They would not even put up any resistance. European militaries are practically non-existent and non-functional without NATO systems, and the US controls it all.

Not to mention the total economic and energy dependence of Europe on the US. The US imposes a blockade on Europe and everything in Europe freezes. Europe has zero strategic autonomy, we gave the last final drop of it up when we let the US bomb Nordstream.

If the US decided to march into Greenland tomorrow and annex it, the EU would put out a strongly worded letter and a week later be back to unequivocally supporting whatever the US does around the world.

“We don’t like what they did, but let’s let bygones be bygones. We cannot afford to alienate our US allies. Us democracies have to stick together. After all, Russia is waging a hybrid war against Europe and plans to invade any day now.”