The Axis powers, originally called the Rome–Berlin Axis, was a military coalition that initiated World War II and fought against the Allies. Its principal members were Nazi Germany, the Kingdom of Italy, and the Empire of Japan. The Axis were united in their opposition to the Allies, but otherwise lacked comparable coordination and ideological cohesion.

The Axis grew out of successive diplomatic efforts by Germany, Italy, and Japan to secure their own specific expansionist interests in the mid-1930s. The first step was the protocol signed by Germany and Italy in October 1936, after which Italian leader Benito Mussolini declared that all other European countries would thereafter rotate on the Rome–Berlin axis, thus creating the term "Axis". The following November saw the ratification of the Anti-Comintern Pact, an anti-communist treaty between Germany and Japan; Italy joined the Pact in 1937, followed by Hungary and Spain in 1939. The "Rome–Berlin Axis" became a military alliance in 1939 under the so-called "Pact of Steel", with the Tripartite Pact of 1940 formally integrating the military aims of Germany, Italy, Japan, and later followed by other nations. The three pacts formed the foundation of the Axis alliance.At its zenith in 1942, the Axis presided over large parts of Europe, North Africa, and East Asia, either through occupation, annexation, or puppet states. In contrast to the Allies, there were no three-way summit meetings, and cooperation and coordination were minimal; on occasion, the interests of the major Axis powers were even at variance with each other. The war ended in 1945 with the defeat of the Axis powers and the dissolution of their alliance. As in the case of the Allies, membership in the Axis was fluid, with some nations switching sides or changing their degree of military involvement over the course of the war.

Particularly within Europe, the use of the term "the Axis" primarily refers to the alliance between Italy and Germany, though outside Europe it is normally understood as including Japan.