This was the German occupation of Austria (12/13 March 1938).
Adolf Hitler had long wished to unify Austria, the land of his birth, with Germany, and had aided the Austrian Nazis agitating for just such an event. However, when the Austrian chancellor, Kurt von Schuschnigg, announced on 9 March 1938 that there would be a plebiscite, on terms unfavourable to the Austrian Nazi party, Hitler saw that he must use military might to force the issue and summoned General Ludwig Beck, the army chief-of-staff, and Generalmajor Erich von Manstein, the deputy chief-of-staff, to urge rapid preparations for a push from southern Germany to Vienna, drawing on some of the planning for 'Otto' (i), by 12 March. Both Beck and von Manstein argued that the only available formations, General Eugen Ritter von Schobertís VII Corps and General Maximilian Reichsfreiherr von Weichsís XIII Corps, together with Generalleutnant Rudolf Veielís 2nd Panzerdivision, were not ready for such an operation, but then improvised the undertaking in just five hours so that the operation orders could be issued during the afternoon of 10 March.
Hitler then forced the replacement of von Schuschnigg by Dr Arthur Seyss-Inquardt, the Austrian Nazi leader, on 11 March with the threat of an invasion unless the plebiscite was cancelled, but next proceeded with the extemporised invasion on 12 March despite the protests of Generaloberst Walther von Brauchitsch, the commander-in-chief of the German army, and of Generalleutnant Max von Viebahn, the Oberkommando der Wehrmachtís director of operations. Both of these officers were unhappy with the nature of German policy, and also feared external intervention should Austria be invaded.
In the event, the invasion was undertaken without encountering any armed opposition, and the British and French refused to do anything but protest, so Seyss-Inquardt was able to declare the Anschluss (annexation) on 13 March, whereupon Austria became the Ostmark of the Greater German Reich.