First part is incorrect. Firstly, Stalin did not invade Poland. There was never a declaration of war, either by the Soviets or by the Polish government. This is because at the time that Soviet troops entered what had up until then been “eastern Poland”, the Polish state had effectively ceased to exist. The government had fled into exile in a neutral country - Romania - and the army had collapsed. There was no more military or civil administration of the country left, it was a complete vacuum. Even western newspapers never referred to it as an invasion at the time, it was simply not seen that way by most people.

Secondly, the territory which was annexed, so-called “eastern Poland” was actually Ukrainian and Belarussian territory that had been stolen by Poland in the 1920 war while Russia was busy fighting a civil war and was unable to prevent the Poles from taking it. (Poland also at that time had attacked and seized territory from Lithuania as well by the way, something many people do not even know about.) According to the treaties signed after the first World War which established the Polish state, its borders were never supposed to go that far east into lands that still had a clear non-Polish majority, even in the 1930s after over a decade of Polish attempts to colonize them. The Soviets were simply reuniting the Belarussian and Ukrainian nations with their lost territories.

Lastly, the only other option the USSR had, seeing as the Poles had previously refused Soviet offers for military aid in case of a German attack, would have been to allow the Nazis to take it all. This would have placed millions more people under their tyranny. Many jewish Poles escaped the Nazi occupied territories by entering the Soviet annexed part where they found for the first time freedom from anti-semitic persecution. Yes it was absolutely the right decision, it would have been in fact a moral crime to not step in to protect those people from the Nazis. As a buffer zone however it did not have that much success.