The general German term Anschluss (literally connection, attachment) is part of the specific political incident Anschluss Österreichs referring to the inclusion of Austria in a "Greater Germany" in 1938. This is opposed to earlier historic Ausschluss meaning the exclusion of Austria from Germany at the creation of Imperial Germany in 1871.
Anschluss was the subject of inconclusive debate prior to the Austro-Prussian War of 1866, whose loss, by Austria, allowed Otto von Bismarck to build the Prussian-dominated German Empire of 1871. Bismarck deliberately excluded Austria because he perceived that the power of the Austrian elite would be a harmful counter-balance to that of the Prussian Junkers in the reunified Germany.
Austria excluded from the new Germany charted a new course within the dual monarchy of Austria-Hungary, attempting to reconcile multiple nationalities within one state. When the multinational empire broke up in 1918 many German Austrians hoped to join with Germany in the realignment of Europe in order to avoid persecution in the new nation states of central Europe. However, after World War I, the Treaty of Versailles in 1919 explicitly vetoed the inclusion of Austria in Germany — largely because France and Britain feared the power of a larger Germany. The fears of many German Austrians were assuaged though because the majority of them remained within the new Austria.
Anschluss of 1938
Inflamed by Hitler's demogogic broadcasts, Austrian Nazis instituted a reign of terror, worsening after election victories in April 1932. To check the power of Austrian Nazis advocating union with Nazi-Germany, Chancellor Engelbert Dollfuss in 1933 had switched to rule by decree, thus establishing an authoritarian regime ending Parliamentarism, and orientated towards fascist Italy. The Nazis' assassination of Dollfuss (July 25, 1934) and many of his supporters facilitated their domination of Austria politically and culturally.
Note: Until the reform of German orthography in 1998, Anschluss was spelled Anschluß; the latter term might thus still be found in older literature. Similarly, Dollfuss is the "internationalized" spelling of Dollfuß. (See also the article on "ess-tzett (ß)".)