Ok.

What was the official reason that the Soviet Union gave for invading Poland in 1939?

I think they said something about the protection of the White or Belo Russian people, but this is from memory. Protection was often used as a euphemism to invade.

That isn't it. Keep trying.

Originally Posted by Doppleganger

That isn't it. Keep trying.

You sure?

The two "justifications" I have managed to find were:

n Sunday, September 17th, Lublin surrendered to the Germans. On the same day at 4.00am, the Red Army invaded Poland from the east to "liberate" and "protect" Belorussians and Ukrainians living in the Eastern Poland from their "Polish Masters"

http://www.achtungpanzer.com/articles/polcamp.htm

and

To justify its actions, the Soviet Union issued a declaration that the Polish state had ceased to exist, and that the Soviet actions were intended to protect the Ukrainians and Belarusians who inhabited the eastern part of Poland.

http://en.wikipedia.org/wiki/Soviet_invasion_of_Poland_(1939)

I've no doubt that the reasons you and Perseus have listed were correct but it isn't the answer I was after.

Keep trying.

Molotov Ribbentrop Pact included a secret protocol, in which the independent countries of Finland, Estonia, Latvia, Lithuania, Poland, and Romania were divided into Spheres of Interest of the Soviets and Germany.

The soviets were claming their land.

Still not right guys.

When the Poles refused to sign an anti-German alliance with the Soviet Union (due to their hatred of communism) the USSR signed the Molotov Ribbentrop Pact and invaded.

You are all coming up with details surrounding the Soviet invasion of Poland and some motivations (excuses) as to why but not the actual reason. It's very simple.

They wanted to take Poland for themselves?