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.
Why was a treaty between the USSR and Japan signed?
The Soviet Union wished to safeguard its eastern border and Japan, bogged down in a war with China sought to secure the northern frontier of Manchukuo against possible Soviet invasion.

Question: How did the union invade?
Self-contained rewrite: How did the Soviets invade Manchuko?