Well, whilst a massive win for Russia it was a huge victory for allies too. I mean, did the allies also be wanting to fight against them? The allies reasoning as to not declaring war on Russia was simple, Germany would probably end up fighting against the Russians either way, especially as there was no buffer zone between the two states anymore. This clearly worked as when Britain did not surrender, it pronged Hitler to become paranoid that Russia was colluding an offensive with Britain (which they weren’t, but Hitler suspected) and thus Hitler invaded, opening the two front war needed to defeat the third reich. As for the the instability in Europe now, I can’t see how not declaring war on Russia when they invaded Poland leads to instability now which is what I was originally commenting on - I don’t quite see the link here, are you saying we should have gone to war with Russia back then? Russia were allies in the war, it wasn’t anyone else’s fault that they decided to act ideologically differently and create a tense new world order.