Yesterday, December 7, was the anniversary of a historic surprise-attack.
Imperial Japan had been waging war since 1937 to take territory from the nation of China, which was itself suffering from a harsh internal war. The United States was trying to embargo raw materials from reaching Japan, and the Japanese Navy was trying to take other regions in the Far East to get better access to those raw materials.
The governments of Germany and Italy had been waging war against the rest of Europe since 1939. An alliance between Germany and Russia had been in place in 1939, but had been thrown aside in 1941.
An unofficial naval war between American Navy ships and German submarines had been ongoing through most of 1940 and 1941. The American ships were going to or from England, and the German submarines were trying to stop anything arriving at England.
These separate threads of conflict, on different parts of the globe, came together in an unexpected way in December 1941.
The Imperial Japanese Navy struck a surprise attack at Pearl Harbor. While doing so, they showed how the aircraft carrier was further-reaching than a heavy battleship.
Within a few hours, the United States Congress declared war with Japan. Within a few days, the nations of Germany and Italy declared war on the United States. (Mostly due to the Tripartite Pact.) Thus, the last major nation to enter a state of war also changed two regional wars into a global war, quickly dubbed "World War II".