They had already done that. At that point, Japan didn't care a whole lot about those territories. The IJN barely bothered to make a defense and the Japanese Imperial Navy opted out entirely.

https://en.wikipedia.org/wiki/Soviet_invasion_of_Manchuria

> At that point, Japan didn't care a whole lot about those territories.

They weren’t really in a position to care any more. I’m fairly certain someone at the top was fairly miffed all their hard won gains were quickly disappearing.

Well, that’s what I meant. I didn’t realize that you didn’t consider invasion of Japanese territory to be a significant entry into the war.

Japan wanted to defend the home island, and if it weren't for the threat of continued nuclear annihilation, they would have. This means either the Soviets would have to figure out a way to launch and maintain an amphibious assault, or they would have to sit on the sidelines.

Japan wanted to negotiate terms. Part of their belief that they could hold out came from the notion that the soviets would come to the negotiating table as a neutral third party.