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From 1228, after of the Livonian Crusade, through the 1560s, Estonia was part of Terra Mariana, established on 2 February 1207 as a principality of the Holy Roman Empire and proclaimed by Pope Innocent III in 1215 as subject to the Holy See. The southern parts of the country were conquered by Livonian Brothers of the Sword who joined the Teutonic Order in 1237 and became its branch known as the Livonian Order. The Duchy of Estonia was created out of the northern parts of the country and was a direct dominion of the King of Denmark from 1219 until 1346, when it was sold to the Teutonic Order and became part of the Ordenstaat. In 1343, the people of northern Estonia and Saaremaa rebelled against German rule in the St. George's Night Uprising, which was put down by 1345. The unsuccessful rebellion led to a consolidation of power for the Baltic German minority. For the subsequent centuries they remained the ruling elite in both cities and in the countryside.

What year did the Livonian Brothers join the Teutonic Order?
1237