OK. After WWI Russia (now the newly minted USSR) was treated as a losing side in the war and new nations were carved from their territory, including Finland, Poland, and the Baltic Nations.
The new USSR wasn’t happy about this and tried to take its part of Poland back, but Poland repelled them. They then signed the Molotov-Ribbentrop Pact in which they agreed to take back ‘their’ part of Poland and let Germany take the rest as well as retaking the Baltic nations. USSR then had a go at Finland, but that didn’t go as planned. Then Germany attacked the USSR taking all of Poland and the Baltics. After years the Red Army won nearly all of the lost territory back.
Somewhat ironically, the USSR was allowed to keep ‘its’ part of Poland as well as the Baltic nations after WWII. They also got to keep the bits of Finland they took in the second Finnish war.
After the war, the Soviets moved many citizens of the Baltic nations out and moved Russians in. The citizens of the Baltic nations do not have fond memories of the Russian, Soviet or German occupations of their nations.
Does anybody ever wonder why the Soviets didn't sign with Britain and France, before Hitler did? The reason was simple. Russia wanted to occupy Poland as a condition of signing an alliance with Britain and France, Britain and France refused, so there was the opening for Hitler.