The alternate history is the Soviets do a ground invasion from the unfortified north. They had broken off their pact with Japan months early and invaded Manchuria just days after the first atomic bomb was dropped, ultimately gaining (and keeping after the Japanese surrender) control of the northern part of the Korean peninsula. Hirohito would have been captured by the Soviets and suffered a fate similar to Puyi's, while Japan itself would have been split up like Germany. This would have been anathema to the Japanese, which is one reason they were scrambling to find a channel to offer unconditional surrender to the US as soon as the Soviets invaded, after having more or less ignored the Hiroshima bombing, their previous channel via Stalin having been cut off.

The US was aware that the Soviets would not renew the nonaggression pact, having negotiated it with Stalin in Yalta in February 1945, and with the Soviet Union denouncing the pact in April. It's possible that the bombs allowed the US to mostly ignore Soviet interests in setting the terms of the surrender, which would have a positive effect, but otherwise, I'm not as sure as most Americans that the atomic bombings were necessary.