World War II effectively stopped the world between 1939 and 1945. To this day, it remains the most geographically widespread military conflict the world has ever seen. Although the fighting reached across many parts of the globe, most countries involved shared a united effort aimed at ending the aggression of the Axis Powers—Germany, Italy, and Japan. Despite the fact that Germany and Japan were technically allies, however, they had vastly different motives and objectives, and their level of cooperation was primarily one of distracting the attention of each other's enemies rather than of attaining any specific common goals. Therefore, most studies of the war cover the conflicts with Germany and Japan separately, dividing treatment of the war between the European and Pacific theaters of operation.
The rise of Nazi Germany and its aggression can be traced directly back to World War I. Following that war, Germany was economically devastated. The Treaty of Versailles unfairly placed the full blame for the war on Germany and demanded heavy reparations payments in return. Although Germany never paid the bulk of these reparations, the treaty humiliated the German people and obstructed the nation's efforts to rebuild itself and move forward economically and technologically. Then, in the late 1920s and early 1930s, the worldwide Great Depression took a further heavy toll on the country.
As resentment and desperation in Germany grew, radical political parties gained in popularity. They ranged from Communists to right-wing nationalists. Among the more extreme activists of the latter category was Adolf Hitler, who had founded the National Socialist German Workers' Party (more commonly known as the Nazi Party) in 1920-1921. By the time of the depression in Germany, Hitler's party had more than 100,000 members and was growing rapidly, and it began participating in parliamentary elections with increasing success. In 1933, Hitler pressured the German president, Paul von Hindenburg, into appointing him chancellor—a position from which he was quickly able to consolidate his power.
By 1935, Germany had ceased to recognize the Treaty of Versailles and all the restrictions that accompanied it. In particular, Hitler announced his intention to fully rebuild Germany's military forces. In 1938, Germany began annexing the territories of neighboring countries, including all of Austria and most of Czechoslovakia. When Germany attacked Poland in September 1939, Britain and France aligned against Germany, and the war began.
Like Germany, Japan was severely affected by the Great Depression. Japan relied heavily upon imported resources and desperately needed more land for its expanding population. Japanese military leaders, who at the time had a strong influence over the civilian government, saw territorial expansion as the best solution. As a result, beginning in 1931, Japanese forces began occupying territory in the Chinese region of Manchuria. By 1937, Japan and China were officially at war. In 1940, the Japanese government announced its intention to establish a "new order in East Asia," under which the region would be freed of Western influence and guided by Japan. In 1940, Japan signed a formal alliance with Germany and Italy, setting the country on a clear course to enter World War II.
In the meantime, the United States, disapproving of Japan's actions, placed a heavy trade embargo on Japan, severely restricting its ability to import oil, scrap metal, and other resources vital to its war effort. Japan saw itself facing an impossible crisis, and without prompt and decisive action, total collapse was inevitable. The action Japan chose was a surprise attack on the U.S. naval base at Pearl Harbor, Hawaii, on December 7, 1941. This action brought the United States into World War II in both theaters, Europe and the Pacific.