Why did Stalin sign the Nazi-Soviet pact with hitler in 1939?

On August 23, 1939, Germany and the Soviet Union sign a non-aggression pact, stunning the world, given their diametrically opposed ideologies. But the dictators were, despite appearances, both playing to their own political needs. i hope its write if not let me know

im pretty sure it has something to do with the end of ww1

it was a act of seld fesnse more over it was a peace treaty between the fogein contunries.