The greatest opportunity to prevent Adolph Hitler's relentless drive of European conquest happened in autumn of 1938.
At that time both Britain and France were still exhausted from World War One, still recovering socially and economically, and very wary of any future conflict, particularly in the face of the newly developed heavy bomber aircraft which could deliver a devastating attack on cities that, up to that time, would have been considered safe from anything but direct invasion by an enemy. The thinking was "the bomber would always get through" to their target, which could very well be your capital city being destroyed on the first day of the war. Additionally neither Britain nor France could afford the costs of rebuilding their military to match German rearmament currently in progress.
In short the desire for peace on the part of Britain and France led British Prime Minister Neville Chamberlain to see accommodation with Hitler over the question of the regions of Czechoslovakia with majority German populations he was demanding be returned to German control was the easiest way to resolve the potential conflict. This was the policy later called "Appeasement". When Chamberlain agreed to this demand Hitler determined the western leaders were too weak to stop him. When both Britain and France subsequently signed a treaty to support Poland militarily if it was attacked by Germany, Hitler thought they were bluffing. Once he attached Poland on Sept. 1, 1939 Britain and France were obligated to declare war on Germany.
The Soviet Union could also have helped rein in Hitler had they not signed the Molotov-Ribbentrop Pact in August 1939 which essentially gave Germany the freedom to attack Poland without having to worry about a Soviet military response. Without that agreement Hitler would most likely not have moved against Poland.
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