Why were there so many Polish governments in exile in WW2? Why did some (one?) choose to fight alongside the country that invaded them (the USSR)?

by corn_on_the_cobh

Seems like there was one in France, that moved to the UK, there was another in Iran? And maybe a third one? I'm not sure. Two is already a lot of governments in exile.

Edit: with further reading, it seems that there was only one government in exile, but I still have a few questions regarding how and why they coordinated with the Soviets (after Katyn especially).

Georgy_K_Zhukov

It looks like you sorted out part of your question, but as to your edit, cooperation was a touchy issue, and the revelations of Katyn actually resulted in a breakdown of relations between the two governments. I touch on this more in this older answer, which also touches on the other aspect of your question regarding Iran.