through Osijek, Mohács and Buda. The natural obstacles in and around the region, especially at the time, included the rivers Sava, Vrbas, Una and Sana, as well as the mountains such as Plješevica, Šator, Klekovača, Raduša, Grmeč, Kozara and Vlašić. Turkish incursions expanded further to the north, and Charles of Styria erected a new fortified city of Karlovac in 1579. In 1580 the Turks responded by declaring the Pashaluk of Bosnia which unified all the Sanjaks, including territory in modern-day Croatia. As a result of the wars and border changes, the Catholic Croat population moved north, and was replaced with Orthodox Serbs. The Bužim fort, under Ottoman control since 1576, was successfully held by the Ottomans in numerous battles (1685, 1686, 1688, 1737) and it was also upgraded (1626, 1834) until their eventual surrender in the 19th century. The building remains to this day as a monument to the Ottoman conquest. Bihać held out longer than Bužim; it was a free royal city and at one time the capital of the Kingdom of Croatia (metropolis et propugnaculum totius regni Croatiae). But, in 1592 the Turkish army of about 20,000 under Hasan-pasa Predojević, an Ottoman vizier, attacked and forcefully occupied Bihać. When the Ottoman Empire lost the 1683–1690 War of the Holy League to the Habsburg monarchy and her allies, and ceded Slavonia and Hungary to Austria at the 1699 Treaty of Karlowitz, the northern and western borders of the Sanjak of Bosnia (corresponding largely to the modern Bosnian borders), became a permanent frontier between the Austrian and Ottoman empires. 19th century In mid-1858, an uprising known as Pecija's First Revolt broke out in the region, resulting from Ottoman pressure against the local Serb populace. It was crushed by December. The Bosnian Frontier, like the rest of Bosnia and Herzegovina, participated in the 1875 – 1878 Herzegovina Uprising. World War II During World War II, Bosanska Krajina was known for its very strong resistance to the Fascist regime of the Independent State of Croatia. The local Serb population in Bosanska Krajina was targeted in the Genocide of Serbs in the Independent State of Croatia by the regime's Croat and Bosniak bands, serving as an overture to future conflicts at the end of the 20th century. The anti-fascist Partisan movement in the Bosanska Krajina region was more ethnically diverse than in any other part of former Yugoslavia during World War II. In