On 3 September 1939, Britain and France declared war on Germany. The two countries had issued an ultimatum to Nazi Germany to stop the invasion of Poland, which had begun two days earlier. Hitler's decision to ignore the ultimatum created the Second World War.
It seems to me this is a pretty good turning point for an alternative scenario, in which Hitler backs down at the last minute. In real life, he was convinced that the ultimatum was an empty threat and that Britain and France were not going to actually declare war. He never for a minute thought that they would actually go through with it.
When the news was brought to him that Britain and France had declared war against Germany, Hitler and Germany's leadership were stunned. I remember reading an account of it somewhere online. The atmosphere was very subdued as the news sunk in. Many of those present had grave misgivings about what had happened. A mood of pervading gloom seems to have settled over those present.
So, what if Hitler believed the threat and backed down? What if he complied with the ultimatum, and halted the invasion? The casus belli (cause for war) would no longer exist, and France and Britain would not declare war on Germany. Of course, it would be a humiliating clime-down. But the war would be averted.
Under this scenario, Germany accepts the ultimatum and makes it known that military action has ceased. The League of Nations convenes to establish a peace settlement. The war against Poland will have lasted just 24 hours.
Thoughts?
This is the ultimatum letter sent by Britain to Germany on 2 September 1939. Failure to respond to the letter resulted in Britain declaring war on Germany just 24 hours later.
http://www.bl.uk/learning/timeline/external/wwiiultimatumletter-tl.jpg