In the 13th century, Poland was suffering from the attacks of Pagan tribes. In response, Konrad I of Masovia hired an army of unemployed crusaders - the Teutonic Order. After the failure of converting the Old Prussians to Christianity, the Order fell into a series of conflicts with the Polish state. The Teutonic Knights invaded the Polish coastal region of Gdańsk Pomerania with the country's main port city of Gdańsk, and therefore took the control of the nearly entire southeastern Baltic Sea coast. The Teutonic Knights continued to occupy the region despite papal verdicts. They remained powerful until 1410, when a combined Polish-Lithuanian army was able to win a decisive victory over the Teutonic Order at Grunwald. As a result, Poland emerged as a major power in Central Europe. Already in the 15th-century, German theologian and writer John of Falkenberg proposed and advocated the genocide of Poles. After the Battle of Grunwald, he suggested that the Polish nation, including the King, should be exterminated either in its entirety, or in majority, and be also reduced to slavery. After Poland challenged his works at the Council of Constance, his views were condemned by Pope Martin V in 1424. He is considered the first writer to formulate the argument in justification of genocide of another nation. During the Thirteen Years' War (1454–1466), Poland regained the territories previously annexed by the Teutonic Knights, and the remainder of the State of the Teutonic Order also became a part of the Kingdom of Poland as a fief.