The Axis powers, originally called the Rome–Berlin Axis and also Rome–Berlin–Tokyo Axis, was the military coalition which 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 far-right positions and general 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 Axis ultimately came to an end with its defeat in 1945.

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

The term "axis" was first applied to the Italo-German relationship by the Italian prime minister Benito Mussolini in September 1923, when he wrote in the preface to Roberto Suster's La Germania Repubblicana that "there is no doubt that in this moment the axis of European history passes through Berlin" (non v'ha dubbio che in questo momento l'asse della storia europea passa per Berlino). At the time, he was seeking an alliance with the Weimar Republic against Yugoslavia and France in the dispute over the Free State of Fiume.

The term was used by Hungary's prime minister Gyula Gömbös when advocating an alliance of Hungary with Germany and Italy in the early 1930s. Gömbös' efforts did affect the Italo-Hungarian Rome Protocols, but his sudden death in 1936 while negotiating with Germany in Munich and the arrival of Kálmán Darányi, his successor, ended Hungary's involvement in pursuing a trilateral axis. Contentious negotiations between the Italian foreign minister, Galeazzo Ciano, and the German ambassador, Ulrich von Hassell, resulted in a Nine-Point Protocol, signed by Ciano and his German counterpart, Konstantin von Neurath, in 1936. When Mussolini publicly announced the signing on 1 November, he proclaimed the creation of a Rome–Berlin axis.

Italy under Duce Benito Mussolini had pursued a strategic alliance of Italy with Germany against France since the early 1920s. Prior to becoming head of government in Italy as leader of the Italian Fascist movement, Mussolini had advocated alliance with defeated Germany after the Paris Peace Conference (1919–1920) settled World War I. He believed that Italy could expand its influence in Europe by allying with Germany against France. In early 1923, as a goodwill gesture to Germany, Italy secretly delivered weapons for the Reichswehr, which had faced major disarmament under the provisions of the Treaty of Versailles.

Since the 1920s Italy had identified the year 1935 as a crucial date for preparing for a war against France, as 1935 was the year when Germany's obligations under the Treaty of Versailles were scheduled to expire. Meetings took place in Berlin in 1924 between Italian General Luigi Capello and prominent figures in the German military, such as von Seeckt and Erich Ludendorff, over military collaboration between Germany and Italy. The discussions concluded that Germans still wanted a war of revenge against France but were short on weapons and hoped that Italy could assist Germany.