There were many events that led up to the start of the second World War. Some historians argue that World II was simply a continuation of World War I. This topic is extremely complex, so I will highlight some of the main causes.
The Treaty of Versailles stated that Germany was to blame for World War I. Germany was ordered to pay reparations to France and to Great Britain. The country had already suffered greatly during World War I. In order to pay the money, they printed a massive amount of German marks. This led to significant inflation, and German money became practically worthless. Some people even burned German marks as fuel because it was cheaper than purchasing firewood. The amount that Germany had to pay was so large that they did not finish payments until 2010. Germany also lost land and territories after the war. The Treaty called for land to be annexed to neighboring countries. The Germans had to de-militarize the Rhineland, which was an area along the border of Belgium. During World War I, the Germans had destroyed coal mines in France. After the war, France ordered Germany to repay back the coal they had destroyed. They requested an enormous amount of coal. At this time, coal was used heavily as fuel. The devastation in Germany after the war led many people to seek a powerful leader to make Germany a strong country again. Adolf Hitler came to power in 1933. With him came a rise of anti-Semitism and a view of the superiority of the "Aryan race." Many Germans were bitter toward countries like Britain and France because they felt that they had been punished for World War I. The late 1910s saw a rise of Communism. The Fascist regimes in both Germany and Italy were strongly anti-Communist.
Italy, Germany, and Japan all expanded their military in the years leading up to World War II. This gave them the capability to wage war against strong military powers, such as the United States and Great Britain.
In 1936, the Germans re-militarized the Rhineland. This violated the Treaty of Versailles. Japan invaded Manchuria in 1937 to gain economic power. Western countries had economic influence in China. Japan wanted this influence to end. They wanted to gain from China's economy. The U.S. stepped in and sought to negotiate with Japan. When they continued to invade other areas, the U.S. stopped all shipments of oil to Japan. The United States government would start shipments again if the Japanese removed their troops, but they refused to do so. Germany annexed Austria in 1938 and invaded Czechoslovakia in 1939.
Despite the tensions between the United States and Japan, it was not until 1941 that war was declared between the two countries. On December 7th, 1941, the Japanese Navy attacked Pearl Harbor. Other Japanese military forces attacked the Philippines and several British territories. The next day, England and Britain declared war on Japan.