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.

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.

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

Stefan Waydenfeld has written:

'The ice road' -- subject(s): Deportations from Poland, Polish Personal narratives, Soviet Prisoners and prisons, World War, 1939-1945

'The ice road' -- subject(s): Concentration camps, Escapes, Polish people, Deportations from Poland, Polish Personal narratives, Concentration camp inmates, World War, 1939-1945, Deportees, Travel, Soviet Prisoners and prisons, Biography, History

The Polish Government in Exile was aligned with the Western Democracies, and Stalin wanted a Polish Government that was Communist.