Following the Slavic migrations to the Balkans postdating the 6th century, Serbs established several sovereign states in the early Middle Ages which at times nominally recognized Byzantine, Frankish and Hungarian overrule. The Serbian Kingdom obtained recognition by the Vatican and Constantinople in 1217, reaching its peak in 1346 as a relatively short-lived Serbian Empire. By the mid-16th century, the entire modern-day Serbia was annexed by the Ottomans, at times interrupted by the Habsburg Empire, which started expanding towards Central Serbia from the end of the 17th century, while maintaining a foothold in modern-day Vojvodina. In the early 19th century, the Serbian Revolution established the nation-state as the region's first constitutional monarchy, which subsequently expanded its territory. Following disastrous casualties in World War I, and the subsequent unification of the former Habsburg crownland of Vojvodina (and other territories) with Serbia, the country co-founded Yugoslavia with other South Slavic peoples, which would exist in various political formations until the Yugoslav Wars of the 1990s. During the breakup of Yugoslavia, the country declared independence in April 1992 as Serbia formed a union with Montenegro , which was peacefully dissolved in 2006. In 2008, the parliament of the province of Kosovo unilaterally declared independence, with mixed responses from the international community.