The main countries in the Allied powers of World War II were the Soviet Union, the United Kingdom, France and the United States. There were several other countries that fought alongside the Allied Powers, but these countries were the major belligerents and provided the vast majority of Allied troops.
The United Kingdom and France first declared war on Germany on Sept. 1, 1939, after Hitler's invasion of Poland. Despite repeated warnings that Germany's actions were in violation of the Treaty of Versailles, Germany continued expanding its territory until the United Kingdom and France declared war. In 1940, Germany's assaults removed France from the major conflict.
Meanwhile, Germany and the Soviet Union had come to an agreement known as the Molotov-Ribbentrop Pact, which stated that neither side would attack the other and the countries would divide Poland between them. Germany broke this agreement in June of 1941, causing the Soviet Union to side with the Allies.
During this time, the United States had maintained an attitude of neutrality. The Japanese Empire was expanding its holdings in the Pacific and wanted to keep the United States from being able to stop it. Therefore, the Japanese military launched an attack on Pearl Harbor on Dec. 7, 1941. On December 8, the United States entered the war in response on the side of the Allies.