Why did Poles join the Red Army and March on Berlin?

by Det_Steve_Sloan

After all, Russia invaded Poland too. Where did the Russians get so many Poles to fight for them, occupiers? Especially after the Katyn Massacre?

Weren't Poles who fought for the USSR traitors?

Georgy_K_Zhukov

This older answer might be of interest although it focuses only on the Anders Army, which was one of two major Polish formations raised within the USSR.