Because of the migrations that followed – such as those of Slavs, Bulgars, Hungarians, and Tatars – the Romanians were organised in agricultural communes (obști), developing large centralised states only in the 14th century, when the Danubian Principalities of Moldavia and Wallachia emerged to fight the Ottoman Empire. During the late Middle Ages, prominent medieval Romanian monarchs such as Bogdan of Moldavia, Stephen the Great, Mircea the Elder, Michael the Brave, or Vlad the Impaler took part actively in the history of Central Europe by waging tumultuous wars and leading noteworthy crusades against the then continuously expanding Ottoman Empire, at times allied with either the Kingdom of Poland or the Kingdom of Hungary in these causes. Eventually the entire Balkan peninsula was annexed by the Ottoman Empire. However, Moldavia and Wallachia (extending to Dobruja and Bulgaria) were not entirely subdued by the Ottomans as both principalities became autonomous (which was not the case of other Ottoman territorial possessions in Europe). Transylvania, a third region inhabited by an important majority of Romanian speakers, was a vassal state of the Ottomans until 1687, when the principality became part of the Habsburg possessions. The three principalities were united for several months in 1600 under the authority of Wallachian Prince Michael the Brave. Additionally, in medieval times there were other lands known by the name 'Vlach' (such as Great Vlachia, situated between Thessaly and the western Pindus mountains, originally within the Byzantine Empire, but after the 13th century autonomous or semi-independent; White Wallachia, a Byzantine denomination for the region between the Danube River and the Balkans; Moravian Wallachia, a region in south-eastern Czech Republic). Up until 1541, Transylvania was part of the Kingdom of Hungary, later (due to the conquest of Hungary by the Ottoman Empire) was a self-governed Principality governed by the Hungarian nobility. In 1699 it became a part of the Habsburg lands. By the 19th century, the Austrian Empire was awarded by the Ottomans with the region of Bukovina and, in 1812, the Russians occupied the eastern half of Moldavia, known as Bessarabia. In the context of the 1848 Romanticist and liberal revolutions across Europe, the events that took place in the Grand Principality of Transylvania were the first of their kind to unfold in the Romanian-speaking territories.