I will give you few more pointers so you can study it in context: - (1917-19) Curzon line, establishment of second Polish republic, establishment of Soviet Uni

I will give you few more pointers so you can study it in context: - (1917-19) Curzon line, establishment of second Polish republic, establishment of Soviet Union, succession of Russian Empire & impact of Brest-Litovsk treaty, succession states of Austro-Hungarian Empire; - (1918-21) Polish-Soviet War, Poland annexing Soviet territories east of Curzon line; - 1925, publication of Mein Kampf; outlining geopolitical ambitions of Germany - Intermarium project, Jozef Pilsudski, geopolitical ambitions of Poland - Polish relations to Germany in 1930s - (1938) Munich agreement, role of Poland - August-September 1939; pay particular attention to timeline of exile of polish government officials, compared to "USSR annexes Poland". But of course, you can ignore all that and pretend that USSR had a capability to start a war at the time (hint: they barely defended themselves, it was huge diplomatic effort to stall the attack on USSR till 1941).