History of Germany during World War I
In November 1918, with internal revolution, a stalemated war, Bulgaria and the Ottoman Empire suing for peace, Austria-Hungary falling apart from multiple ethnic tensions, and pressure from the German high command, the Kaiser and all German ruling princes abdicated. On 9 November 1918, the Social Democrat Philipp Scheidemann proclaimed a Republic, in cooperation with the business and middle classes, not the revolting workers. The new government led by the German Social Democrats called for and received an armistice on 11 November 1918; in practice it was a surrender, and the Allies kept up the food blockade to guarantee an upper hand. The war was over; the history books closed on the German Empire. It was succeeded by the democratic, yet flawed, Weimar Republic.
German Unity Day
An alternative choice to commemorate the reunification could have been the day the Berlin Wall came down: 9 November 1989, which coincided with the anniversary of the proclamation of the German Republic in 1918, and the defeat of Hitler's first coup in 1923. However, 9 November was also the anniversary of the first large-scale Nazi-led pogroms against Jews in 1938 ("Kristallnacht"), so the day was considered inappropriate as a national holiday. (See November 9 in German history.) Therefore, 3 October 1990, the day of the formal reunification, was chosen instead and replaced the "Day of German Unity" on 17 June, the national holiday of the Federal Republic of Germany from 1954.