Starting in the 12th century, the Margraviate, later Electorate, of Brandenburg was in conflict with the neighboring Duchy of Pomerania over frontier territories claimed by them both, and over the status of the Pomeranian duchy, which Brandenburg claimed as a fief, whereas Pomerania claimed Imperial immediacy. The conflict frequently turned into open war, and despite occasional success, none of the parties prevailed permanently until the House of Pomerania died out in 1637. Brandenburg would by then have naturally have prevailed, but this was hindered by the contemporary Swedish occupation of Pomerania, and the conflict continued between Sweden and Brandenburg-Prussia until 1815, when Prussia incorporated Swedish Pomerania into her Province of Pomerania.