Following World War I, the German word Anschluss was used to denote the union of Germany and Austria (short for "Anschluß Österreichs an das Deutsche Reich", i.e. Austria joining, or becoming part of, the German Reich). This was a basic idea of the Pan-German nationalist movement, and a first attempt, in 1918, to perform it was ineffective due to the World War I peace treaties. Now the Anschluss is mainly associated with 1938 when Nazi Germany incorporated Austria as the Ostmark ("Eastern Marches"). Austria became independent again after World War II.
The 1938 Anschluss, which is the most common usage of the term, was the result of a threefold process, a combination of:
- Quasirevolutionary seizure of power (Anschluss from the base),
- Imperialistic intervention by the German Reich (Anschluss from outside)
- An apparently legal seizure of power (Anschluss from the top).
The roots of the Anschluss idea are found in the 19th century pan-German movement that, under the leadership of Prussia, led to the union of the many small independent German states, but with the exclusion of the German parts of the Austrian-Hungarian empire. Austria had previously been part of the mediaeval Kingdom of Germany and its shadowy successors "The Holy Roman Empire of the German Nation" and the German Confederation (1815-67).
After World War I, Austria-Hungary disintegrated and its provinces declared their independence. Many believed that the small German-speaking rest of Austria (often called "Restösterreich") could not survive on its own. Consequently, the constituting provisional Austrian general assembly proclaimed, on 12 November 1918, a new republic with the name Deutsch-Österreich ("German-Austria"), and, in Article 2, declared it part of the German republic.
This first Anschluss, however, could not be realized because it was explicitly forbidden by the peace treaties of Versailles (28 June 1919) with Germany and of Saint-Germain-en-Laye (10 September 1919) with Austria. The latter also prohibited the name "Deutsch-Österreich", so Austria had to become the "Republik Österreich".
In spite of this, nationalist political groups both in Germany and Austria continued to demand the Anschluss (Anschlussbewegung).
In 1931, another attempt at closer collaboration, the project of a tariff union with Germany, failed due to French opposition. The international court in The Hague ruled against it.
In an agreement between Hitler and Schuschnigg (July 1936), Hitler had to acknowledge Austria's independence and to officially give up his influence over Austria's national socialism, while Schuschnigg had to promise a "German" politics and taking in the national opposition. The parties to the treaty were soon accusing each other of violating it.
The Anschluss of Deutschösterreich to the German republic was first proclaimed by the (provisional) Austrian national assembly (12 November 1918). This was led by Victor Adler, who was Secretary of State for Foreign Affairs under the interim government of Karl Renner. He died on 11 November, the day the war ended, and a day before the proclamation. Adler had been one of the coauthors of the Linz Program (1882), which called for the Germanization of Austria.
Attempted customs union
There was an attempt, in the early 1930s, to create a customs union between German and Austria, to create larger markets. Nevertheless, external powers, seeing this as an attempt to circumvent the Treaty of Versailles, prevented it. 
Attempted customs union
The process accelerated with the Austro-German Agreement of July 1936, with a secret annex that gave additional power to the Austrian Nazis. They steadily increased subversion and terrorism throughout 1937, and Austrian police captured documents indicating they planned to stage a revolt in the spring of 1938, which could provide a pretext for German intervention. Austrian Chancellor Kurt von Schuschnigg had, in 1937, tried to get Britain to declare that it would guarantee Austria's independence. Without that, his opposition gradually weakened. 
Nazis request Schuschnigg-Hitler meeting
Former German Chancellor Franz von Papen had been a special representative to von Schuschnigg. Von Papen revealed that one of the captured documents had called for his own killing by German agents, again as a pretext for intervention. Ironically, von Papen had escaped death in the Night of the Long Knives purge in 1934. Hans Lammers informed him, on 4 February, that he was fired, along with Constantin von Neurath and others who did not give total support to Hitler, such as War Minister Werner von Blomberg and Army Chief of Staff Werner von Fritsch. Von Papen began keeping, in Switzerland, secret copies of his correspondence with Hitler. The most powerful German diplomat in Austria was not von Papen, but Wilhelm Keppler. He would become the first Reich Commissioner for Austria, shifting to Slovakia in 1939.
Von Papen, however, asked Schuschnigg, in early February, to meet Hitler.
The Anschluss 1938
Austria and Germany merged in 1938. The process required a seizure of power by local Nazis, pressure and intervention from Nazi Germany, and changes in the Austrian government.
This process was set in motion when Schuschnigg had to visit Hitler in Berchtesgaden. Under heavy pressure he had to concede an amnesty for national socialists, and to accept Seyß-Inquart as Innenminister (minister for interior affairs) in his cabinet (appointed 16 February).
During the weeks following this visit, control and power shifted gradually from the government to the Nazi Party, which took over key positions in the bureaucracy. Beginning around 20 February, Nazi rallies took place throughout the country, to which the Vaterländische Front responded in kind.
Since Schuschnigg knew that the workers' leaders wanted an independent Austria, he announced, on 9 March, a plebiscite to take place on 13 March, expecting about 65 to 75 percent votes in support. The German reaction can be considered an indication that this was a realistic assumption.
11 March 1938
Anschluss from the base
The 11 March 1938 was characterized by Nazi demonstrations everythere and an openly shown confidence of victory. By command of the its central administration, the regional sections of the NSDAP occupied the local centres of power, a task completed in the evening by about 21 o'clock. Also in the evening, the first arrests of officials of the "authoritarian regime" and of Jews took place.
Anschluss from outside
At the same day, a series of ultimata, presented by Hermann Goering via telephone, and communicated by the ministers Seyß-Inquart and Glaise-Horstenau led first to the cancelling of the plebiscit, then to the resignation of Schuschnigg. Though hesitatingly, Bundespräsident (Federal president) Miklas had to appoint Seyss-Inquart as chancellor (around 22 o'clock) and finally, near midnight, to accept the list of ministers presented to him.
12 March 1938
In the morning of the next day, the first German troops entered Austria, and in the afternoon Hitler's way from Braunau (at the border) to Linz was a triumph, because the frustrated population welcomed him in a spirit of hope for a better future.
Also in the morning, Heinrich Himmler and his staff landed in Vienna. In what followed, his force of 12,000 German police were brought in position faster than the military troops.
13 March 1938
Though originally Hitler did not intend to fully perform the Anschluss at once, he took advantage of the people's acclamation and the weak foreign reactions. Austria became part of the Third Reich, until 1942 as Ostmark, then as "Donau- und Alpenreichsgaue" (the Danube and Alps provinces). Both "Mark" and "Gau" are old German words denoting a geographical region or territory.
Anschluss from the top
On 13 March, the new NS controlled Ministerrat accepted a Bundesverfassungsgesetz (Federal Constitutional Law) on the Wiedervereinigung.
Seyss-Inquart now led the Austrian government (Landesregierung), Wilhelm Keppler represented the German ministries (Reichsbeauftragter), while Josef Buerckel (Reichskommissar), directly responsible to Hitler, had the most influence.
The plebiscite of 10 April 1938
Soon the Anschluss was supported by public figures (like Karl Renner) and, in particular, by a formal declaration of the Austrian Catholic church (March 18). The plebiscite was rescheduled for 10 April and thoroughly prepared by the NS administration. Massive propaganda "by words" in all available media was accompanied by "propaganda by deeds", i.e., by political actions improving the economic situation and supporting the unemployed.
Shirer observed the referendum itself, which recorded a 99.75% agreement in Austria, was not conducted in a fair manner. The Social Democrats and Schuschnigg's own Christian Socials did not campaign. In the voing place he viewed, it was easy to see how an individual voted. The vote was announced 30 minute after the close of voting, before the votes could have been counted. . The result of 99.7 percent participation (with 200,000 people excluded) and 99.6 percent of voting "yes" is therefore no surprise.
Britain filed a formal diplomatic protest, but took no other action.
Shirer, reporting from Vienna, reported the antisemitism of the Austrian Nazis as worse than anything he had seen in Germany, characterized by looting and sadism. Reinhard Heydrich, assisted by Adolf Eichmann, set up an Office of Jewish Emigration, by which would-be emigrants could buy permission to leave. They also created Mauthausen Concentration Camp in Austria proper, to avoid the difficulty of transporting Jews to Germany.
Schuschnigg had been put under house arrest from March 12 through May 28, and then taken to a hotel for the next 17 moths, where he was required to clean the guards' latrines. He and his wife were later taken to concentration camp, and were freed on 4 May 1945.
- Victor Adler, The Original Nazis
- Richard J. Evans (2003), The Coming of the Third Reich, Penguin, ISBN 1-59420-004-1, p. 235
- Joachim Fest (1973), Hitler, Harcourt Brace Jovanovich, pp. 567-568
- William Shirer (1960), The Rise and Fall of the Third Reich, Simon & Schuster, pp. 322-324
- Shirer, p. 350
- Shirer, p. 351