The Independent State of Croatia (Serbo-Croatian: Nezavisna Drava Hrvatska, NDH; German: Unabhngiger Staat Kroatien; Italian: Stato indipendente di Croazia) was a World War II-era puppet state of Nazi Germany and Fascist Italy. It was established in parts of occupied Yugoslavia on 10 April 1941, after the invasion by the Axis powers. Its territory consisted of most of modern-day Croatia and Bosnia and Herzegovina, as well as some parts of modern-day Serbia and Slovenia, but also excluded many Croat-populated areas in Dalmatia (until late 1943), Istria, and Meimurje regions (which today are part of Croatia).

During its entire existence, the NDH was governed as a one-party state by the fascist Ustaa organization. The Ustae was led by the Poglavnik, Ante Paveli. The regime targeted Serbs, Jews and Roma as part of a large-scale campaign of genocide, as well as anti-fascist or dissident Croats and Bosnian Muslims. According to Stanely G. Payne, crimes in the NDH were proportionately surpassed only by Nazi Germany, the Khmer Rouge in Cambodia and several of the extremely genocidal African regimes.In the territory controlled by the Independent State of Croatia, between 1941 and 1945, there existed 22 concentration camps. The largest camp was Jasenovac. Two camps, Jastrebarsko and Sisak, held only children.The state was officially a monarchy after the signing of the Laws of the Crown of Zvonimir on 15 May 1941. Appointed by Victor Emmanuel III of Italy, Prince Aimone, Duke of Aosta initially refused to assume the crown in opposition to the Italian annexation of the Croat-majority populated region of Dalmatia, annexed as part of the Italian irredentist agenda of creating a Mare Nostrum ("Our Sea"). He later briefly accepted the throne due to pressure from Victor Emmanuel III and was titled Tomislav II of Croatia, but never moved from Italy to reside in Croatia.From the signing of the Treaties of Rome on 18 May 1941 until the Italian capitulation on 8 September 1943, the state was a territorial condominium of Germany and Italy. "Thus on 15 April 1941, Paveli came to power, albeit a very limited power, in the new Ustasha state under the umbrella of German and Italian forces. On the same day German Fhrer Adolf Hitler and Italian Duce Benito Mussolini granted recognition to the Croatian state and declared that their governments would be glad to participate with the Croatian government in determining its frontiers." In its judgement in the Hostages Trial, the Nuremberg Military Tribunal concluded that NDH was not a sovereign state. According to the Tribunal, "Croatia was at all times here involved an occupied country".In 1942, Germany suggested Italy take military control of all of Croatia out of a desire to redirect German troops from Croatia to the Eastern Front. Italy however rejected the offer as it did not believe that it could on its own handle the unstable situation in the Balkans. After the ousting of Mussolini and the Kingdom of Italy's armistice with the Allies, Tomislav II abdicated from his Croatian throne: the NDH on 10 September 1943 declared that the Treaties of Rome were null and void and annexed the portion of Dalmatia that had been ceded to Italy. The NDH attempted to annex Zara (modern-day Zadar, Croatia), which had been a recognized territory of Italy since 1920 and long an object of Croatian irredentism, but Germany did not allow it.

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.