Following Swedish military successes, including an invasion of Prussia, Frederick William I was forced to take Prussia as a fief from the Swedish king in the Treaty of Königsberg on 7 January (O.S.) / 17 January (N.S.) 1656. Before, Frederick William I had held that duchy as a fief of the Polish king. In Königsberg, the "Great Elector" further had to meet the Swedish demands for Pillau and Memel, promise financial and military aid, and surrender half of the port duties to Sweden.