I agree with you that an antifascist alliance should have been formed and could have stopped Hitler much sooner and with much less death.

You seemed to imply that the USSR gladly made a peace treaty with Hitler at the onset instead of trying to gather allies to fight Hitler first.

Let me know which hyperspecific term for invading another country

Imperialism. Unless the USSR expelled or tried to exterminate the citizens of conquered territory, that would be settler colonialism.

For more information on the terminology and why it’s important: On ‘Arab Colonialism’