In an effort to enlarge his dominions, Konrad unsuccessfully attempted to conquer the adjacent pagan lands of Chełmno in Prussia during a 1209 crusade with the consent of Pope Innocent III.

Subjected to constant Prussian raids and counter-raids, Konrad now wanted to stabilize the north of his Duchy of Masovia in this fight over the border area of Chełmno.

Thus in 1226, Konrad, having difficulty with constant raids over his territory, invited the religious military order of the Teutonic Knights to fight the Prussians,[6] as they already had supported the Kingdom of Hungary against the Cuman people in the Transylavanian Burzenland from 1211 to 1225.

In view of an imminent Prussian invasion, Konrad supposedly signed the Treaty of Kruszwica in 1230, according to which he granted Chełmno Land to the Teutonic Knights and the Order of Dobrzyń.

They had ten children: Konrad is considered by Poles to be responsible for Teutonic Knights' control of most of the Baltic coastline, undermining Polish authority in the area.

After the Thirteen Years' War in the 1466 Second Peace of Thorn, the Polish King Casimir IV Jagiellon gained control over the Chełmno Land as part of Royal Prussia.