Leo Szilard was a Hungarian physicist and the driving force in trying to get an atomic bomb project begun in the United States. The problem was that most physicists at the time did not believe it was possible to harness atomic energy. Atomic fission had been proven possible but creating a process to convert atomic energy into a weapon was still only a theory, and not even a universally accepted theory. To try and convert that theory into a usable item would require time, personnel, equipment, and, above all, vast amounts of money and other resources.
At the time (summer of 1939) there was no atomic research going on anywhere in the United States. With war on the horizon there were literally hundreds of projects that were in need of personnel and funding. Convincing FDR this one project was deserving of special consideration was the key.
Now why exactly would the President of the United States agree to commit vast sums (which could have gone to more concrete weapons programs) to try and build something other scientists didn’t believe could be built solely on the word of a Hungarian scientist he’d never heard of?
That was why Einstein was so important. No, he was never a member of the Manhattan Project and had no direct hand in developing the bomb. Yet his contribution was absolutely vital. His name and his fame were what convinced FDR to okay the original project. Without that there would have been no atomic bomb project in the United States, or at best a much smaller commitment made after America was at war. Most certainly atomic bombs would not have been created in time to be used in World War Two and depending on the decision to form such a project and its results it is VERY possible the United States would not have been the first country to develop atomic weapons.
World War Two would still have been an Allied victory even if at higher cost and longer duration in the Pacific. Germany would NOT have built them as Hitler had no faith in them and did not divert enough resources for his project to have a viable chance to succeed. What happens after World War Two is the real question. If not the United States the USSR would have been the likely creator of them.
Even if no one believes they can be built and the US does not have atomic weapons to act as a deterrent how long will peace in Europe endure?