The Western Powers had already determined the occupation zones for defeated Germany. Ike didn't see the point in spending lives taking something he'd have to turn over to the Russians... Russia did agree to enter the war against Japan, and it did so. One might argue that it accelerated its entry as it learned of the Bombs so as to have a seat at the table.

And having seen the damage from the Tokyo firebombing all the Bomb provided was efficiency. The US had already come up with the proper means of erasing cities.