The order theoretically lost its main purpose in Europe with the Christianization of Lithuania. However, it initiated numerous campaigns against its Christian neighbours, the Kingdom of Poland, the Grand Duchy of Lithuania, and the Novgorod Republic (after assimilating the Livonian Order). The Teutonic Knights had a strong economic base which enabled them to hire mercenaries from throughout Europe to augment their feudal levies, and they also became a naval power in the Baltic Sea. In 1410, a Polish-Lithuanian army decisively defeated the Order and broke its military power at the Battle of Grunwald (Tannenberg). However, the capital of the Teutonic Knights was successfully defended in the following Siege of Marienburg and the Order was saved from collapse.