Short answers, pulled from the story.
The Anschluss was the annexation of the Federal State of Austria into Nazi Germany, completed on the 12th of March 1938. German troops crossed the border unopposed after Chancellor Kurt Schuschnigg resigned under threat of invasion, and Austria ceased to exist as an independent state.
Hitler, an Austrian German by birth, wrote in Mein Kampf in 1925 that he would unite Austria and Germany by any means possible. The first point of the 1920 National Socialist Program demanded unification of all Germans in a Greater Germany, and the "Heim ins Reich" policy sought to incorporate ethnic Germans living outside Germany into the Reich.
The official plebiscite of the 10th of April 1938 recorded 99.7% support, but historians treat this figure with caution. Around 360,000 people, roughly 8% of eligible voters, were stripped of their voting rights before the vote. Political scientist Eric Voegelin, who fled Austria after the annexation, wrote that "there was not much doubt that in 1938 a majority of Austrians did not favor a union with Germany."
Persecution began immediately on the 12th of March 1938. Jews were forced onto the streets, expelled from professions and schools, and subjected to the Nuremberg Laws from May 1938. By the end of 1941, 130,000 Jews had left Vienna. Of the more than 65,000 Viennese Jews deported to concentration camps, fewer than 2,000 survived.
Goering was the loudest voice within Nazi leadership calling for annexation, driven primarily by economic pressure. Hitler's Four Year Plan was falling behind its steel production targets, and in a secret speech before German industrialists in April 1937, Goering stated that annexing Austria, rich in iron, was the only solution. British historian Ian Kershaw wrote that Goering, far more than Hitler, made the running throughout 1937 in pushing for an early resolution of the Austrian question.
From 1949 to 1988, many Austrians relied on the "victim theory," the idea that Austria was the first country forcibly occupied by Nazi Germany, to avoid a full reckoning with domestic complicity. The theory supported the 1955 Austrian State Treaty restoring sovereignty. By 2008, a survey found 82% of Austrians considered themselves a distinct nation; in 1987, only 6% identified as German.