"Its the Lesser Evil"

It was extremely obvious war was coming with the Germans eventually, as much as the USSR would've loved to avoid it to build up their capacity more, I'm sure. Germany was filled with anti-communist propaganda, they arrested and killed a lot of their own communists, a lot of their antisemitic propaganda associated the Jews with communists (the Judeo-Bolshevik conspiracy theory that continues in conservative circles to this day). Germany and Japan literally made an anti-communist pact in 1936 that Italy later joined. They were on opposite sides of the Spanish Civil War, etc.

They tried to ally with the West first but weren't able to because the West didn't like communism, and preferred to just go with the policy of appeasement to avoid a war without having to ally with the USSR. So they ended up making a non-aggression pact with Germany to protect themselves. It's the same reason they expanded their border area, to protect themselves and give them the time for the inevitable future conflict they knew was coming with the Nazis. Remember, they were still recovering from World War 1 and various civil wars. At first, they tried to do it through a treaty so their troops could move through Poland voluntarily to help defend against Germany, but they refused.

And let's not forget that the US does the same thing, but worse, expanding their military bases not for defensive reasons but for imperialist reasons, all around the world.

It marked the withdrawal of German troops to the demarcation line secretly agreed to in the Molotov–Ribbentrop Pact, and the handover of the city and its fortress to the Soviet Red Army.

You do realize non-aggression treaties are not the same as being allies, right? And it was broken only 2 years later. And a condition of the pact was for the nazis to withdraw from the area?

Also there's the fact that you're using fucking Wikipedia as a source. How pathetic.