Located between the regions of Greater Poland and Mazovia, it was the tribal area of the Goplans (Latin Glopeani, which roughly means "residents of Lake Gopło") with the political center in Kruszwica. It was connected to Greater Poland from the 10th century, and to the Duchy of Masovia from 1138.[1] In 1231, the duchies of Sieradz and Łęczyca, had been formed from a part of the state.[2] In 1233 it became an independent duchy under Duke Casimir I, which, due to further divisions of inheritance (1267 and 1314), fragmented into the sub-duchies of Brześć, Inowrocław and Gniewkowo.[3][4] After the unification of part of the Polish duchies to form the Kingdom of Poland under King Władysław I Łokietek, it lost its sovereignty after 1306 and became its vassal. In the Polish–Teutonic War, the sparsely populated area of the duchy was occupied by the Teutonic Order in 1332. The Teutonic Order returned the duchy to the Polish crown in the Peace Treaty of Kalisz (1343).
The duchy was confiscated as a settled fiefdom by the Polish crown towards the end of the 14th century and was given direct administrative control in the kingdom in the course of the 15th century in the form of two incorporated voivodships (with seats in Brześć Kujawski and Inowrocław with a joint local state parliament (Sejmik) in Radziejów). The area belonged to Poland until the Partitions (1772, 1793, 1795).
In the following centuries, the memory of the duchy was only preserved in the names of the voivodeships and in the titulary of the Polish rulers. King Władysław II Jagiełło claimed the following territories in his title:
Wladislaus dei gracia Rex Polonie, nec non terrarum Cracovie, Sandomirie, Siradie, Lancicie, Cuyauie, Lituanie princeps supremus, Pomeranie, Russieque dominus et heres, etc.
