The Anschluss (German: [ˈʔanʃlʊs] ^ⓘ, or Anschluß, lit. 'joining' or 'connection'), also known as the Anschluß Österreichs (pronunciation^ⓘ, English: Annexation of Austria), was the annexation of the Federal State of Austria into Nazi Germany on 12 March 1938.

The idea of an Anschluss (a united Austria and Germany that would form a "Greater Germany") arose after the 1871 unification of Germany excluded Austria and the German Austrians from the Prussian-dominated German Empire. It gained support after the Austro-Hungarian Empire fell in 1918. The new Republic of German-Austria attempted to form a union with Germany, but the 1919 Treaty of Saint Germain and Treaty of Versailles forbade both the union and the continued use of the name "German-Austria" (Deutschösterreich); they also stripped Austria of some of its territories, such as the Sudetenland. This left Austria without most of the territories it had ruled for centuries and amid economic crisis.

By the 1920s, the Anschluss proposal had strong support in both Austria and Germany, particularly to many Austrian citizens of the political left and center. One vehement supporter was prominent Social Democrat leader Otto Bauer, who served as Austria's Foreign Minister 21 November 1918 – 26 July 1919. Support for unification with Germany came mainly from the belief that Austria, stripped of its imperial land, was not viable economically. Popular support for the unification faded with time, although it remained as a concept in the contemporary Austrian political discourse.

In January 1933, Adolf Hitler (born in Austria) rose to power in Germany. From then on, desire for unification could be identified with the Nazi regime, for whom it was an integral part of the Nazi "Heim ins Reich" ("back home to the realm") concept, which sought to incorporate as many Volksdeutsche (ethnic Germans outside Germany) as possible into a "Greater Germany". Nazi Germany's agents cultivated pro-unification tendencies in Austria, and sought to undermine the Austrian government, which was controlled by the Fatherland Front, which opposed unification. During an attempted coup in July 1934, Austrian chancellor Engelbert Dollfuss was assassinated by Austrian Nazis. The defeat of the coup prompted many leading Austrian Nazis to go into exile in Germany, where they continued their efforts to unify the two countries.

On 5 November 1937, Hitler informed his military aides that he would annex Austria and "Czechia" to the German Reich. When Austrian chancellor Kurt Schuschnigg met Hitler in Berchtesgaden on 12 February 1938, he was presented an ultimatum and forced to appoint Arthur Seyss-Inquart as minister of the interior and security. On the eve of 9 March 1938, Schuschnigg announced that there would be a referendum to be held on 13 March to decide between a possible union with Germany or the maintenance of Austria's sovereignty. Schuschnigg expected to win a clear majority to face the Nazi challenge, but the Nazis refused and demanded the appointment of a new cabinet under Seyss-Inquart. Under the threat of military occupation, Schuschnigg resigned and Hitler had the German Army cross the border into Austria on 12 March, unopposed by the Austrian military. A plebiscite was held on 10 April, resulting in 99.7% approval.

The idea of grouping all Germans into one nation-state had been the subject of debate in the 19th century from the dissolution of the Holy Roman Empire in 1806 until the break-up of the German Confederation in 1866. Austria had wanted a Großdeutsche Lösung (greater Germany solution), whereby the German states would unite under the leadership of the Austrian House of Habsburg. This solution would have included all the German states (including the non-German regions of Austria), but Prussia would have had to accept a secondary role. This controversy, called dualism, dominated Prusso-Austrian diplomacy and the politics of the German states in the mid-nineteenth century.

In 1866 the feud finally came to an end during the Austro-Prussian War in which the Prussians defeated the Austrians and thereby excluded the Austrian Empire and German Austrians from Germany. The Prussian statesman Otto von Bismarck formed the North German Confederation, which included most of the remaining German states, aside from a few in the southwestern region of the German-inhabited lands, and further expanded the power of the Kingdom of Prussia. Bismarck used the Franco-Prussian war (1870–1871) as a way to convince southwestern German states, including the Kingdom of Bavaria, to side with Prussia against the Second French Empire. Due to Prussia's quick victory, the debate was settled and in 1871 the "Kleindeutsch" German Empire based on the leadership of Bismarck and Prussia formed—this excluded Austria. Besides ensuring Prussian domination of a united Germany, the exclusion of Austria also ensured that Germany would have a substantial Protestant majority.

The Austro-Hungarian Compromise of 1867, the Ausgleich, provided for a dual sovereignty, the Austrian Empire and the Kingdom of Hungary, under Franz Joseph I. This diverse empire included various different ethnic groups including Hungarians, Slavic ethnic groups such as Croats, Czechs, Poles, Rusyns, Serbs, Slovaks, Slovenes, and Ukrainians, as well as Italians and Romanians ruled by a German minority. The empire caused tensions between the various ethnic groups. Many Austrian pan-Germans showed loyalty to Otto von Bismarck and only to Germany, wore symbols that were temporarily banned in Austrian schools and advocated the dissolution of the empire to allow Austria to rejoin Germany, as it had been during the German Confederation of 1815–1866. Although many Austrians supported pan-Germanism, many others still showed allegiance to the Habsburg monarchy and wished for Austria to remain an independent country.