World War II (1939-1945) began with the German invasion of Poland, and its subsequent division between the Soviets and the Germans. On the Eastern front, the war began with full-time Japanese vs. Chinese war. The Soviets then tried to invade Finland in what is known as the Winter War. They failed.
The next step was the invasion of Western Europe by Hitler and his Nazis. He invaded Belgium, Denmark, Norway, Holland and northern France. (Southern France had a government sympathetic and controlled by Hitler). The Germans also tried to takeover Britain in the Battle of Britain—an aerial battle in 1940. The Germans met with defeat in Britain. Italy then entered on the side of Hitler and took another part of France. The Germans then invaded Yugoslavia.
Throughout WWII Spain under the dictatorship of Franco remained neutral. The other countries that remained neutral throughout WWII were Ireland, Portugal, Sweden, Andorra, Liechtenstein, the Vatican City and Switzerland.
The two main events that led to the globalization of WWII were the attack on Pearl Harbor and Operation Barbarossa.
Operation Barbarossa—June 22nd, 1941—was the attack by the Germans on their supposed ally the Soviets. This is significant because it made Russia a real power in the war, and also because it was the largest campaign in military history.
The Attack on Pearl Harbor by the Japanese occurred on December 7th, 1941. There is controversy over whether or not President Franklin D. Roosevelt had encouraged and/or desired the attack. Following the attack, FDR had to declare war on Japan. However, he wanted to fight the Germans. Coincidentally, Hitler declared war on the U.S. on December 11th, 1941, just four days after the attack on Pearl Habor. He had practically “saved the day” for FDR.
Therefore, Operation Barbarossa led to Russia being a full-time power in the war, and the Attack on Pearl Harbor brought America into the war in both the East and West.
At this point then Italy and Germany (Austria was annexed to Germany) were against the Allies (FDR (U.S.A.), Churchill (Britain) and Stalin (Russia)). Japan was fighting against America and China. There were several other countries all around the world fighting. The war was now officially global.