What if Great Britain or France dropped an atomic bomb on Berlin in 1939, effectively ending the war before it began?
I think that if we are to answer this question, we must first focus on who may have possibly been the most influential scientist and theorist of the 20th century: Albert Einstein. Historically, Einstein wrote and endorsed a 1939 letter to Franklin D. Roosevelt urging him to research and advance technology in the field of bombing. Einstein himself had been compelled to write such a letter by physicists Leo Szilard and Eugene Wigner, who were concerned with Germany possibly working on a powerful new type of bomb since German scientists had recently split the Uranium atom. Einstein, a German citizen himself, had been living in the United States since 1933 soon after Hitler's rise to power. Just imagine that for whatever reason, he decided to move to Britain to be closer to home and instead of meeting Szilard and Wigner at the eve of WWII actually did so closer to 1935...
In reality, by 1944 Einstein already feared the consequences of the use of atomic weapons writing "when the war is over, then there will be in all countries a pursuit of secret preparations with technological means which will lead inevitably to preventative wars and to destruction even more terrible than the present." (quoted from a letter written to his friend Niels Bohr).
After moving to London, Einstein is visited by Szilard and Wigner and told about the recent splitting of the Uranium atom. Einstein is encouraged to write and sign a letter to Prime Minister Churchill, and he decides to not only encourage research into advanced weaponry but asks them to entice and/or smuggle out the scientists who successfully split the uranium. Churchill fearing Germany's continued aggression agrees and with the assistance of MI5 manages to retrieve these scientists. Those rescued include: Otto Hahn, Lise Meitner, and Fritz Strassmann who in our actual timeline discovered what would come to be known as fission based on a research paper published by Ida Noddack in 1934. WIth all the scientists researching in a remote location in Britain, the bomb is developed at a much faster rate. Eventually in secret, Britain manages to develop the first atomic bomb, and soon thereafter Hitler invades Poland. Churchill makes the decision to use the weapon on Berlin, the bomb is dropped and Germany immediately surrenders after its capital city and central leadership is obliterated. The war is stops before it even begins- or does it?
WWII was in some ways caused by old grudges the previously victorious Entente Powers of WWI helped to perpetuate, by the harsh penalties and war reparations imposed on Germany when the treaty was signed. Prior to 1939's invasion of Poland, Germany was already allied with both Russia and Italy. Let's say Germany surrenders along with her allies, but just like the aftermath of the bombing of Nagasaki and Hiroshima- suddenly everyone wants in on the atomic bomb action. The problem is that this time, there are more parties involved that haven't had to face a four year long war that actually drained and destroyed much of their resources. If anything Italy, Russia and even Japan are at the height of their powers.
The mere proximity of the countries alone would have made atomic bomb attacks a lot easier and more quickly accomplished.
This would cause a powder keg of a chain reaction, by 1945 all the major European powers would have created their own version of the atomic bomb. I can imagine a wasteland of all the major cities being hit by such a weapon- St. Petersburg, Moscow, Rome, Paris, London, Tokyo, the list goes on. The nuclear apocalypse that was so hardly fought from happening during the Cold War would come early. But where does this leave the United States? Without the Pearl Harbor rallying cry, and the fears raised by the bomb, the nation would remain staunchly isolationist. More than likely, Roosevelt would not be re-elected for a third term and real life opponent Wendell Willkie (running on a much stronger isolationist and anti-New Deal platform) would have won the election of 1940.
The real Wendell Willkie joined the Roosevelt administration as an "unofficial" diplomat and travelled to many of the allied countries. He returned from these travels with a new found resolve that the concept of imperialism was flawed. Without these travels, Willkie himself may never have come to realize these views. In addition, without the constant stress of travel during Roosevelt's 3rd term he may have lived well past his actual death in 1944.
I imagine the US would have still surged out of the Depression, with fears abroad being the catalyst to strengthening the military at home. The United States would have developed the same weapon roughly in line with the countries abroad, but the added benefit of an entire ocean of separation would keep the many of the affairs of Europe out of American society. With all the destruction overseas, the United States would remain the only power able to continually develop and improve on atomic weapons. By the early 1950s, many of the same people that were agitators of the second Red Scare would still be around. The focus of course would not be on communism. As stated before without a 4 year long war to wear out most of the rest of Europe both financially and structurally, communism would have never risen as it did when Russia and the US were the only two left standing.
Joseph McCarthy was the individual who famously started the Red Scare by claiming to have a list of known communists in the state department. McCarthy in this timeline, would instead shift his focus to having a list of known "European spies" in positions of power and rally an already isolationist country into a violent fervor to close down borders and continue to strengthen the military. McCarthy himself was also close friends with the Kennedy family and his campaigns were in part funded from the family's deep pocketbook. It was actually John F. Kennedy's father's hope that McCarthy would eventually run and quite possibly win as the first president to identify himself as Catholic, thus paving the way for the Kennedy family to eventually get a son in office. Assuming that after President Wendell Wilkie won in both the 1944 and 1948 elections, McCarthy is now viewed as a real contender for the office by the election of 1952. With Wilkie's endorsement, McCarthy easily wins and with Nixon in tow as Vice-President the time is ripe for an expansionist movement.
McCarthy was a true master politician and was able to easily manipulate the American people into a frenzy over fears of communist influence. He used many of the tactics of Yellow Journalism which is ironically most associated with the brief imperialist movement within the US in the very later 19th and very early 20th century.
By having its borders closed by President Wilkie in 1950, the United States would continue to have a rise in patriotism. McCarthy could easily use this to bring back something that brought the nation together prior to WWI: imperialism. The argument would of course be to further solidify the continent of North and South America against the by now complete distrust of all of Europe. After diplomacy fails to essentially force South American nations into "puppet kingdoms" of the McCarthy administration, war would be declared. The continent would be invaded and those countries that continue to resist would eventually face the ultimate doom: the atomic bomb. After South America is brought to its knees, the entire continent would fall under American control. By the election of 1954, McCarthy's popularity is at an all time high and he easily wins. We now have a radically different world, the went in a very dark direction.
So, going back to the initial question: I rather think that if such a situation had occurred things may have been a whole lot worse than they ultimately ended up. This of course is looking at it from a bleaker perspective, but I find it difficult to see it going anywhere else. Would peace be possible if such a weapon had been used so soon in the conflict? I rather think it would be, but the problem is that WWII was what really helped strengthen the relationship between world powers. As difficult as it is to believe, a lot of the people in the United States prior to Pearl Harbor weren't interested in getting involved in a conflict they understandably felt didn't concern them. If a full out atomic war was actually occurring throughout Europe by 1945, this would only deepen America's resolve to stay out of foreign affairs. Where do you think things would have gone if an atomic bomb had been dropped on Germany in 1939?
Thank you for reading, I'd like to stress that most (if not all) of my sources are going to come from Wikipedia. This understandably should not be considered the most trustworthy of sources, however I myself am not a historical expert merely someone who is deeply interested in the topic. I would also like to say as an admirer of history, this article was in no way written to defame or otherwise harm real historic individuals or downplay the importance of historical events. The idea is entertainment, and as my original abridged article at Foss' Flicks stated: the intention is not to insult but to create wonder on one of humankind's favorite questions: What if?