Who declared war on the Axis?

The Allies, primarily the United States, the Soviet Union, and the United Kingdom, declared war on the Axis powers during World War II. The U.S. formally entered the war after Japan's attack on Pearl Harbor on December 7, 1941, leading to a declaration of war against Japan the following day. Subsequently, Germany and Italy declared war on the U.S., prompting the U.S. to respond with declarations against them as well. Thus, the Allies united in their fight against the Axis powers throughout the war.