Between 1409-1411 Janusz I continued his support to Władysław II Jagiełło in the great war against the Teutonic Order and fielded a banner of cavalry to aid the Polish King. In the village of Czerwińsk nad Wisłą, Janusz I designated the place of concentration of the united Polish-Lithuanian army. From there him, at the head his army organized their squadrons of knights and went to Grunwald, where on 15 July 1410 took place the battle. Janusz I then participated in the rest of the campaign. Władysław II, as way to recompensate his fidelity, give to him the Teutonic castles of Nidzica, Ostróda and Olsztyn. However, this acquisitions weren't permanent, because seven months later (1 February 1411) after the sign of the Peace of Thorn, he was forced to return the castles to the Teutonic Order.