It is surprising that I found myself reading a Conroy alternative history, as I found his 1901 to be historically absurd. But I picked up 1945 and started reading anyway.
The point of divergence is that, shortly before Emperor Hirohito was able to deliver his address calling on the people of Japan to surrender to the Allies, he was kidnapped by a clique of military officers. These men overthrew the government and planned to continue the war, even though the U.S. had threatened to nuke every Japanese city, one at a time.
Conroy contrives a scenario whereby America can no longer practically use atomic weapons against Japan and must proceed with Operation Downfall -- the planned invasion of Japan in the Fall of 1945 and Spring of 1946. His rationale is not really all that good, but it is good enough to get the reader to the rather intriguing scenario in which Downfall actually takes place.
Conroy is a worse historian than Harry Turtledove, but a better novelist. Overall, it's worth reading if you're into alternate history.