Over a pint my brother and I pondered whether once they were invented, it was inevitable that the Atomic bombs would be dropped.
As of OTL July 1945, yes.
Yes.Particularly.
-If FDR had lived longer, would the bombs still have been dropped ?
Not much. Any U.S. President in that position would have made the same decision. Not using a weapon that had cost $2 billion to make and could save hundreds of thousands of American lives? Unthinkable.-How much of the bombs being dropped was down to Truman?
However, there are other ways the Bomb might not have been used then and there.
For instance, if Stauffenberg's bomb killed Hitler, and the war in Europe ended in 1944, then
1) Allied effort switched to Asia defeats Japan sooner, before the Bomb is ready.
1)a) Soviet forces overrun Manchuria, Korea, and Sakhalin, and threaten Hokkaido. The Japanese hardliners' fantasy was that the U.S. would invade the Home Islands, encounter fanatical resistance including mass banzai attacks by millions of civilians, and would be so traumatized by the casualties that it would agree to a conditional peace. But I don't think even they believed that would work with Stalin.
2) With the Nazi menace dead, and no threat of a Nazi bomb, and with six-eight months work still to do, many Manhattan District scientists refuse to finish the job. A lot of them never wanted to build the Bomb in the first place.