The war grew, with Polish forces from the Duchy of Masovia and Gdańsk striking the nearby Teutonic fortifications. Teutonic forces were on defense, waiting for reinforcements from Germany, which arrived in the summer of 1520. In July, the Teutonic army started an offensive, attacking Masovia, Warmia, and Łomża territories, laying siege to Lidzbark Warmiński. In August another group of German reinforcements attacked Wielkopolska, taking Międzyrzecz. The Germans took Wałcz, Chojnice, Starogard Gdański, and Tczew and started a siege of Gdańsk, but they retreated when faced with Polish reinforcements and plagued by financial troubles (German reinforcements, mostly mercenaries, refused to fight until paid). Polish forces retook Tczew, Starogard, and Chojnice. The Teutonic Knights retreated towards Oliwa and Puck, pursued by Polish forces. The Polish side was then struck with financial troubles, and the "pospolite ruszenie" forces were also tired. The Teutonic Knights seized their chance and launched a counteroffensive, taking Nowe Miasto Lubawskie and approaching Płock and Olsztyn. Olsztyn was successfully defended by the Poles under the command of Nicolaus Copernicus.