The Soviets wanted to eliminate the ethnic Poles, get revenge for the Soviet loss in the Polish-Soviet war in 1919-1920, and acquire Polish land.

One of the causes of the famine in the Soviet Union in 1946 was the World War II.

If you mean the invasion in World War II under the Nazi-Soviet pact, it was certainly not because Poland was "weak." Poland had just defeated the Soviets in the Polish-Soviet War in 1919-1920. Instead, the Soviets, along with the Germans, wished to destroy all ethnic Poles. Stalin also held a grudge against the Polish for his court-martial after the Polish-Soviet war. The Soviets were responsible for nearly as many deaths as the Germans, and they didn't respect the rights of POWs.

the Soviet Union was Comunist and America wasn't

Tadeusz Laskowicz has written:

'Wspomnienia zza krat' -- subject(s): Biography, Personal narratives, Polish, Polish Personal narratives, Political prisoners, Prisoners and prisons, Soviet, Soviet Prisoners and prisons, World War, 1939-1945