The Manhattan Project fails to produce a workable nuclear weapon in time for use against Japan. What are the consequences? Two seem t be
1) A land invasion of Japan proves necessary.
1.1) Japan is shattered even more completly the OTL.
1.2) Japan suffer devisting depopulation, survivors post war reconciliation proves difficult
1.3) Allied forces suffer signficant causulties during land invasion, more vetran troop transfered from Europe.
2) Stalin is embolded by no nuclear bomb is prepared to escalate the Berlin Blockade,
2.1) First into limted offensive to take Berlin,
2.2) Could he be prepared to escalate into into general war,
2.3) What preperation/differences would Stalin take?
Are the any other divergencies
Ok, rathar than "prove impossible", The Manhatten Project proves unsuccessful causing a significant delay in the production of a workable weapon. (Say from 45 until 55). The importance of the Einstein letter is missed. Objectors to Oppenheimer succeed and a less able,driven,visionary leader is appointed (Who?)