Why were the USSR and Germany as allies at the first stage of the war?

This could not be called a full-fledged alliance, because everyone defended their interests.

The Soviet Union realized that the Western countries, Britain and France, with the tacit consent of the United States, were pushing Hitler to go to war with the USSR.

Stalin offered to create a collective security system in Europe and defend Poland, but Britain and France refused, so the Soviet Union went to rapprochement with Germany in order to secure their country. That is why, at first, they acted as allies.

Stalin agreed to this deal, knowing full well that sooner or later a war with Germany would begin. But thanks to this alliance, it was possible to return the primordially Russian lands of the west of Ukraine and Belarus.