Italy was looking for more resources through a non-colonized country, Abynissia, invading it and violating international treaties and war atrocities were committed on each side.
German re-militarization of Rhineland
Germany violates the treaty of Versailles and moves troops back into the Rhineland, against the law due to its proximity of France. Germany refuses to listen to allied pleas to respect the treaty
Annexation of Austria by Germany
Germany violates the treaty of Versailles and gains control of Austria, contributing to Germany's plan to create a greater Germany and get more living space.
Annexation of Sudetenland
Because many Germans lived in Sudetenland, Hitler viewed it as belonging to Germany and made Czechslovakia give that part of the country to Germany to allow more living space and a larger population for Germany.
Britan declares war on Germany
Two days after Germany attacked Poland, Britan declared war, not willing to allow Germany to keep gaining power and territory in Europe. Germany was directly breaking the treaty of Versailles and was a direct threat to Britan, This was the beginning of WWII.
France declares war on Germany
As with Britan, France had to respond to Germany repeatedly gaining power and breaking the treaty of Versailles, as well as sharing a border with Germany. With France so close to Germany they were in great danger of an attack anyways and had to take action.
German Invasion of Norway (and Denmark)
Germany invaded Denmark and Norway to stop the allies from blockading the baltic sea, Germany's only access to the ocean was in a small passage between Denmark and Norway and Sweden. Fearing their oceanic trade routes being cut, the germans controlled the passage and the countries around it.
Battle of Britan
7/10/1940 - 10/31/1940
The Luftwaffe (German airforce) begins to bomb and attack British cities and important areas to weaken the country. This was the first major air conflict of the war.
Attack on Pearl Harbor
The Japanese viewed the United States as in their way of control over the Pacific, so without formally declaring war, they launched a suprise attack on Pearl Harbor and destroyed a good part of the Unites States naval fleet. The Harbor was well defended but not expecting the attack, quickly being decimated by Japanese aricraft and submarines.
The Fall of Hong Kong
On the same morning as the attack on Pearl Harbor, Japan captured the city of Hong Kong against international law, which was defended by British, Chinese, Indian, and Canadian soldiers.