By Gordon McDaniel, M.L.S.
After the Battle of Mohacs in 1526, the Austrian Habsburg Empire found itself having a common border with the Turkish Ottoman Empire. The Turks had defeated the Hungarian army in that battle, during which the Hungarian King died, and the remaining Hungarian lands, now restricted to Royal Hungary (mostly Slovakia and western Hungary) and Croatia, soon became hereditary lands of the Habsburg dynasty.
In order to stem the Turkish tide, the Habsburg court created in 1528 a zone in Croatia ruled directly from Vienna (instead of through the Hungarian nobility), in which land ownership was dependent on providing military service. These frontier troops were predominantly Serb, Croat and German. This zone was called the Militär Grenze (Military Frontier, Vojna krajina).
A period of reconquest and Habsburg expansion followed the failed Siege of Vienna in 1683. By the early 19th century, the boundary between Habsburg and Ottoman lands had settled more or less on the line of the Una, Sava and Danube Rivers. Most of this land had been historically Hungary, but again the Austrian court expanded its system of military frontier zones to include parts of Slavonia, Vojvodina or Banat, and Transylvania. The frontier zones in Transylvania reverted to Hungarian civilian rule in 1851, but those in Croatia, Slavonia and Vojvodina/Banat remained in one form or another until 1881.
References for further reading:
Historical Atlas of East Central Europe, Paul Robert Magocsi. -- Seattle : University of Washington Press, 1993 (paperback, 1995).
The Austrian military border in Croatia, 1522-1747, Gunther E. Rothenberg. -- Urbana, University of Illinois Press, 1960.
The Military Border in Croatia, 1740-1881: a study of an imperial institution, Gunther E. Rothenberg. -- Chicago, University of Chicago Press, 1966