Following a long campaign of terror by Austrian Nazis, including bombings and violent disorder throughout the country, Chancellor Schuschnigg agreed to attend a meeting with Hitler on February 11th 1938 at Berchtesgaden.
At the meeting Schuschnigg was harangued by Hitler and presented with an agreement to sign which would place Austrian Nazis into prominent govenrment positions, including Dr Seyss-Inquart as Minister of the Interior (who would therefore have control of the Austrian police). Under threat of armed invasion from Hitler, Schuschnigg agreed to sign the document and promised to secure its endorsement by the Austrian President, Miklas. The president finally agreed to sign the document 3 days later realising the threat of armed invasion was genuine.
On 20th February 1938 Hitler gave a speech in the Reichstag proclaiming that all German people living outside the German borders (i.e. those in Austria and Czechoslovakia) should have the right to protection from the German Reich. This led to further violent pro-Nazi demonstrations in Austria, and Seyss-Inquart, now in charge of the police did nothing to quell the violence.
By March 9th Schuschnigg had decided as a last desperate measure to hold a plebiscite to ask the Austrian population to declare whether or not they wanted a free independent Austria. This was to be held on Sunday March 13th. News of the proposed poll sent Hitler into a rage and he ordered his generals to prepare a plan for military action against Austria. On March 11th Hitler issued orders for the occupation of Austria under the name Operation Otto. The German-Austrian border was closed and German troops began to mass along the frontier.
Hitler then sent a message via Seyss-Inquart to Schuschnigg instructing him to call off the plebiscite or face military action. Schuschnigg with little choice, agreed to do so, yet was further pressed by Hitler and Goering to resign and announce Seyss-Inquart as his successor.
Seyss-Inquart thus took over as Austrian Chancellor, despite the initial refusal by President Miklas to endorse the appointment. Under his leadership, the German Army were then invited to march into Austria under the pre-text that they were there to help quell unrest in the country.
Hitler was reasonably confident that the main European powers, Italy, France and Britain would do nothing to oppose the invasion, as Mussolini had indicated that he was not interested in what happened to Austria, France was undergoing a change of Government and Britain had indicated that it would do nothing in the event of military intervention by Germany.
On March 12th 1938 Hitler triumphantly entered Austria, initially staying in Linz, near his birthplace. On the same day the new Austrian government passed a law for total Anschluss to make Austria a province of the German Reich. Hitler entered Vienna on March 14th to a rapturous welcome by cheering crowds.
To cement the illusion of legality, a plebiscite was held in Austria on April 10th for the public to decide whether or not to endorse the union with Germany (encouraged by propaganda, coercion and fear of reprisals) - the official "yes" vote was announced as 99% and Austria was finally absorbed into the German Reich.