Even if they don't have American nukes behind them?
Given what we've seen in Ukraine, I agree that Eastern European countries can likely take on Russia in conventional warfare. But that's not the only thing on the table.
They don't need American nukes, UK and France have nukes.
Somehow I don't think that either UK or France would be willing to get into a nuclear exchange with Russia over Poland or Finland, especially considering the relative sizes of the nuclear arsenals involved, and then also how much less concentrated the Russian population is.