The Japanese empire extended into mainland east Asia at the time. There’s no reason why the soviets couldn’t have made meaningful entry into the war against Japan through Manchuria and the Korea peninsula.

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.

One can debate whether the Soviets would have been able to invade the Home Islands, or whether this would have been necessary to press the Cabinet into surrendering.

But it does seem strange to say they were not in a position to make a "meaningful" entry by that point, given the order of battle employed in the Manchuria campaign (per WP):

    1,577,725 troops
    27,086 artillery pieces
    1,152 rocket launchers
    5,556 tanks and self-propelled guns
    3,721 aircraft
    Mongolia:
    16,000 troops

The allied invasion of Japan was going to consist of over 6 million allied soldiers.

1.5 million was not anywhere near enough. The Soviets fielded 2x that in the Battle of Berlin alone!

Maybe they weren't going to help invade mainland Japan straight away but there were still plenty of Japanese troops in China that needed to be dealt with.

https://en.wikipedia.org/wiki/Second_Sino-Japanese_War