Conversation so far:
when did the soviet union declared war on japan and invade manchuria
On August 9, 1945, Stalin unilaterally abrogated the USSR's Neutrality Treaty with Japan and declared war on Japan, beginning the Soviet–Japanese War.
What was the USSR and Japan neutrality treaty?
The Soviet–Japanese Neutrality Pact was a non-aggression pact signed on April 13, 1941, two years after the conclusion of the Soviet-Japanese Border War.

Question: Why was the treaty formed?
Self-contained rewrite: Why was a treaty between the USSR and Japan signed?