Answer :

In 1939 (Nazi-Germany) and Soviet Union agreed to split Poland.

The treaty that specified the conditions of this split is refered to as the Molotov–Ribbentrop Pact, after the Foreign ministers of the two countries that signed it. It was signed on 23. August 1939 and it was at the time officially claimed that it only determined that the two countries would not engage in aggression with each other. At the time the split of Poland (and the division of other countries such as Lithuania and Estonia between the two powers) was kept secret.

Hitler & Joseph Stalin made a pact in which they agreed not to attack each other, but to attack POLAND and split it between the two countries.

Spoiler alert: Hitler breaks his pact & decides to attack the USSR*

In one word your answer is: Poland.

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