I’ve always wondered, why the Allies declared war on Germany, but not the Soviet Union for attacking Poland. Besides obvious reasons like not going to war with two rather “strong” nations why havent they done so?
It was framed as a “liberation”. USSR claimed that Polish state has collapsed so therefore it doesnt have to respect deal which upheld the border. At the same time, they framed it as a move to defend Ukrainians and Belorussian, minorities in Poland but main nations of their internal republics. And of course, they said that it was not an “aggression” but simply a move to take over the control and establish peace.
There was no declaration of war and no polish commander gave an order to fight the Russians so there was a great deal of confusion. Their “offensive” lasted a week - that gives you the scale of the conflict.
Allied governments did send notes of disapproval though
Media called it a dirty move - a backstab. But at the same time, there were nodes of agreement as to the anti-german nature of the move.
To answer your question - mostly what you said but there was a lot of confusion