The Oder-Neisse line has been accepted to be the eastern German boundary by all post-war German states (East and West Germany, as well as reunited Germany)[20][circular reference], reneging on all plans to (re-)expand into or (re-)settle territories beyond this line. The Old Prussians were conquered by the Teutonic Knights in the 13th century, and gradually assimilated over the following centuries; the Old Prussian language was extinct by the 17th or early 18th century. Henry Cord Meyer, in his book "Drang nach Osten: Fortunes of a Slogan-Concept in German–Slavic Relations, 1849–1990" claims that the slogan Drang nach Osten[21] originated in the Slavic world, and it also was more widely used than in Germany.[21]