Multiple different answers
Explanation:
I could possibly argue for:
1. Britain and France not fulfilling their promises towards Italy in the Treaty of Versailles
2. Hitler's rise to power because of poor German treatment in WW1
3. The Nazi-Soviet joint invasion of Poland
4. Failure of the League of Nations
5. Aggression of Japan
Now let me explain all the stuff above:
1. Italy's poor treatment
In WW1, Italy was theoretically on the Central Power's side since they were "allied" with Germany. However, Italy fought for the Allied Powers side after Britain and France told Italy that they would get a lot of stuff (land and money) in the peace treaty. In the end, Italy got literally nothing, and not enough reparations were paid to Italy to save their really really bad economy. Then this guy named Benito Mussolini came around and said "I can fix everything!" and invented Fascism, meant to unite all Italian peoples from the days of the great Roman Empire (you can kinda already see the problem: Nazi Germany had territory that was a part of the Roman Empire). Mussolini's fascism would inspire Hitler to create Nazism. Italy defeated Ethiopia shortly after even though it took them like too long to defeat bow and arrows with Tanks and bombs, and basically everyone hated Italy after that.
2. Hitler's Rise to Power
Germany took the blame for everything that happened in WW1 -- including the absolute terrible British and French generals and their horrific tactics at the Somme, Passchendaele, the Marnes, and Gallipoli -- and therefore had a whole bunch of stuff taken away from them. They had their once arguably most powerful military reduced to a simple home defense of 100,000 men, couldn't have an air force, had to demilitarize the Rhineland, and had to pay like a couple hundred billion dollars that wasn't even fully paid until like 2010. Then this guy named Hitler comes around and says "I can fix everything!" and created Nazism based on Fascism and then he took part of Czechoslovakia, Austria, Yugoslavia, and half of Poland before finally Neville Chamberlain and Charles de Gaulle finally declared war on Germany.
Ok this explanation is gonna be kind short now thank god this looks like a quora answer
3. The Nazi-Soviet joint invasion of Poland
Neville Chamberlain and Charles de Gaulle stated that if Germany invaded Poland, they would declare war on Germany. And then Hitler and Stalin invaded Poland, so yeah
4. Failure of League of Nations
WOW! WORLD PEACE! is what everyone thought of when League of Nations came about, except for world peace never happened since the League of Nations COULD ABSOLUTELY DO NOTHING. Like they were so useless even I could have been a better political influence. They could not enforce any rules set against Germany and Italy for their aggression since they did not have a military (thankfully, the UN has one today). And Germany and Italy did their stuff invading all these countries, leading to British and French declaration of war.
5. Japanese Aggression
Were I to be an honest American in 1940, I would think that this was just 2 different wars - an chaotic European Anglo/Franco- Prussian/Italo War and a Sino-Japanese War. I personally believed that it was only a world war until Japan invaded Pearl Harbor, causing US to declare war on Japan and Japanese ally Germany to declare war on US. These two declarations of war tied Europe, Asia, and North America together in a conflict. (Theoretically you could argue against me that 1939 Canada joining WW2 as a cause since different continents fought each other)
