In 1410, John Sokol assisted the Polish-Lithuanian coalition against the Teutonic Knights in the Battle of Grunwald. Together with Stanislav of Dobrá Voda, he commanded the fourth Polish banner of St. George, consisting of about 1,500 horsemen. After the victory of the Poles, he participated in the conquest of further castles and became the administrator of Radzyń Chełmiński Castle. After the Peace of Thorn, the castle was returned to the Teutonic Knights.