Siemowit IV, Duke of Masovia

1373/74), and as a result of the partition of Masovia between him and his older brother Janusz I after the death of their father on 16 June 1381, Siemowit IV finally obtain the totality of his domains: Rawa, Płock, Sochaczew, Gostynin, Płońsk and Wizna.

However, the late King had made arrangements among the Lesser Poland nobility who guaranteed their support to his eldest daughter and heiress Mary and her husband Sigismund of Luxembourg.

Unexpectedly, in Buda the Dowager Queen Elizabeth of Bosnia decided to change the decision of her husband and accepted the reign of her youngest daughter Jadwiga over Poland instead of Mary and Sigismund, who remained rulers of Hungary.

This decision caused that several supporters of Siemowit IV to hope that he could marry the young Queen (despite the fact that she was already betrothed to William of Habsburg) and in this way, both factions could reconcile and in addition this union with the old Piast dynasty could further legitimize the Angevin rule.

This proposal quickly gained widespread acceptance, and only thanks to the courageous intervention of the voivode of Kraków John of Tęczyn, who advised them to abstain from any decision until the arrival of Jadwiga, the idea was abandoned.

Consequently, and after a final rejection in October 1384 from the powerful Lesser Poland nobility to a marriage with Jadwiga, the Masovian Duke adopted a different tactic: if it was impossible for him obtain the crown, he decided to win all the territory possible.

Siewomit, after adding Łęczyca to his territory, decided to undertake peace negotiations with Jadwiga, which ended successfully on 12 December 1385 with the signing of a treaty, under which Siemowit IV returned all the lands taken by him in exchange for the sum of 10,000 silver marks, and most important, he relinquished all his claims to the Polish crown and paid homage to the Queen Jadwiga and her new husband and King, Jogaila, from which he received the Duchy of Belz as dowry of Princess Alexandra of Lithuania, Jogaila's sister, who married Siemowit IV as a gesture of reconciliation between both parties.

This was expressed in his frequent contacts with the Hungarian King Sigismund, who, wishing to drag a Polish vassal to his side gave the Masovian Duke the rich prebends from the Bishopric of Veszprém and other possessions across Hungary.

Another gesture of friendship with King Władysław II was noted when he used Siemowit IV's daughters into political marriages and the support given to his son Alexander in his Church career.

In 1387, Siemowit IV married Alexandra (died 20 April 1434), a Lithuanian princess, daughter of Algirdas, Grand Duke of Lithuania, and his second wife, Uliana of Tver.