World War II began in September 1939. But the reason the War lasted five and a half years were the policies of England, United States and Russia in the 6 years leading up to the outbreak of the War. Despite all of the tricks of the Nazi Party in Germany, they did not hide their ideology of the Germans being a master race, the Jews being vermin, and their goals of expansionism to the West and East of Germany.
However, following the deep human cost in World War I, nearly every country was not anxious to get involved in a world conflict again. That natural inclination was correct. The cost of war is truly horrible, and World War I was terrible. But tyrants with massive lethal power don’t sit quietly unless they have tremendous fear of being annihilated. That did not occur with the Nazi party. Actually, the reverse happened.
The more threats came from Germany, and the more Germany instituted racial laws like the Nuremberg Laws, the more the Western world doubled down on NOT getting involved in German affairs. They were determined to NOT get embroiled in messy wars that they thought were not relevant to them. The winds of isolationism were extremely strong in the United States, but even in England too. Appeasement was the order of the day as late as 1938, when Neville Chamberlain went to meet with Adolf Hitler.
The Soviet Union was even worse. They colluded with Germany, thinking that by joining in with Germany in their common interest to expand their countries, they would avert any threats from Germany. The net result of all of these actions by the Western powers and the Soviet Union was tens of millions of non-combatants and soldiers killed in battle by the German war machine. It could have been a short and quick war in 1936 or 1937, but the Allies were actually ill prepared to battle then and continued their policies of appeasement while the German war machine kept on building up.
There is one solution to defeating a war machine of Nazi-like forces. Destroy it. Finish it off. Take away land, and subjugate their population to live according to your rules. Otherwise, kick them out. That is language that is understood by the tyrants and it works. Germany is today a country that is part of the Western nations. Japan is an ally of the United States against the Chinese threat. There are millions of Arabs today who would be happy to not be killed like Hamas terrorists. Even Iran has a majority that does not want to be dominated by a radical Muslim fringe. But they will only be willing to not threaten the free world when they understand that the alternative to maintaining their threats on the free world is total destruction.