On February 24, 1918, shortly before the Brest-Litovsk treaty was signed, an Estonian republic had been proclaimed by the Maapäev, a popularly elected assembly, which however was opposed by the landowning German aristocracy. The following day did German troops invade.
Soon under the command of Rüdiger von der Goltz, who arrived after having secured the victory for the Germany-loyal government in the similarly newly independent Finland, the German troops occupied all of what would become Estonia and Latvia.
On November 5th, 1918, a temporary Regency Council was created of a Diet (General Provincial Assembly) for the former Baltic German provinces of Courland, Livonia and Estland, i.e. today's Latvia and Estonia, as the first step of forging a united "Baltic Duchy" under German aegis. The Head of State was to be Adolf Frederick, the Duke of Mecklenburg-Schwerin, not as a sovereign monarch, but as the subordinate to the Kaiser Wilhelm II, similar to other dukes and grand dukes of the German Empire. It had also been considered to establish a personal union with Kaiser Wilhelm in his role as King of Prussia.
Duke Adolf Frederick never assumed office and instead a Regency Council consisting of four Baltic Germans, three Estonians and three Latvians was appointed.
The representative assembly of the Duchy was to consist of the native, but ethnically German, nobility, which however called upon Kaiser Wilhelm II to recognize the Baltic provinces as a monarchy, and a German protectorate, instead of a duchy.
The flag was a black Scandinavian cross on white background.
The defeat of Germany in the World War, November 1918, and in the Baltic the defeat of the aristocracy-led Landeswehr supported by numerous German volunteers, November-December 1919, made the United Baltic Duchy irrelevant. The Baltic states were re-established as republics.