Siemowit IV (Ziemowit IV), also known as Siemowit IV the Younger (pl: Siemowit IV M?odszy; ca. 1353/1356 – 21 January 1426), was a Polish prince member of the House of Piast from the Masovian branch, from 1373/74 Duke of Rawa, and after the division of the paternal inheritance between him and his brother in 1381, ruler over Rawa, P?ock, Sochaczew, Gostynin, P?o?sk and Wizna, since 1386 hereditary Polish vassal, since 1388 ruler over Belz, during 1382-1401 he lost Wizna and during 1384-1399 and 1407-1411 he lost Zawkrze, during 1384-1399 he lost P?o?sk, taken by the Teutonic Order.
Already during his father's lifetime, Siemowit IV received his own district, Rawa Mazowiecka (ca. 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, P?ock, Sochaczew, Gostynin, P?o?sk and Wizna.
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Siemowit IV chosen to be opposed to his older brother Janusz I in the relations with the Polish Kingdom - in particular, when he attempted to obtain the royal crown. One year after the acquisition of his own domains, King Louis of Poland and Hungary died (10 September 1382), and with this emerged the opportunity to place his candidacy to the crown, supported by the Greater Poland and Kujawy nobility (centered around the powerful Bartosz Wezenborg). However, the late King had made arrangements among the Lesser Poland nobility who guaranteed the support to his eldest daughter and heiress Mary and her husband Sigismund of Luxembourg. Without waiting for a favorable settlement, in January 1383, Siemowit IV marched to Greater Poland at the head of his troops, marked the beginning of the Greater Poland Civil War.
Unexpectedly, in Buda the Dowager Queen Elizabeth of Bosnia decided to change the decision of her husband and accepted to 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 hoped that he could married 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 legitimized the Angevin rule.
The first step to implement this plan was the formal candidacy of Siemowit IV to the royal crown. To this end, in a meeting of nobles and gentry at Sieradz the Archbishop Bodzanta of Gniezno, one of his leading supporters, proposed Siemowit IV's candidacy to the throne. This proposal quickly gained widespread acceptance, and only thanks to the courageous intervention of the voivode of Kraków John of T?czyn, who advised them to abstain from any decision until the arrival of Jadwiga, the idea was abandoned. The opposition of Lesser Poland to the candidacy of Siemowit IV was probably associated with the fear of the growing role of Greater Poland under an eventual rule of the Masovian Duke. Another argument against this was the emerging idea of the union with the Grand Duchy of Lithuania.
This events didn't discouraged Siemowit IV, who was determined to obtain the crown, even by force. Probably with the knowledge and consent of Archbishop Bodzanta, he attempted to abduct Jadwiga and marry her, in a desperate act to win the crown. When the Lesser Poland nobility knew of his intentions, they close the gates of Wawel to Bodzanta's men, among them was hidden Siemowit IV. They also warned Jadwiga, who remained at the court of her mother until was secured to travel.
Despite the defeat of his ambitious plans, Siemowit IV continued his efforts to obtain the Polish throne. For this purpose, after burning the property of his political opponents in Ksi, he went back to Sieradz, where a part of the local nobility proclaimed him King of Poland. This time, however, the congress lacked of real authority, and for this reason, he delayed his expected coronation, trying to conquer the country by force. After a disastrous campaign and a failed siege to Kalisz (Siemowit IV was able to obtain only Kujawy), some of his supporters decided to sign an armistice on 29 September 1383.
Better advantage of the ceasefire to the Polish was the join of Hungarian troops to the country under the personal command of Sigismund of Luxembourg. The combined attack of Hungarian-Polish forces caused that Siemowit IV resigned from further fighting, moreover when his brother Janusz opted for the recognition of Jadwiga as Queen.
The defeat of Siemowit IV reduced considerably the number of his supporters. 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.
This time the fight took place successfully for Siemowit IV: by the end of 1384 he could conquer czyca. However, he soon realized that, given the power of the Polish-Lithuanian union (confirmed in the Union of Krewo), his forces are too small. Finally, he decided to make 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.
Siemowit IV's final testimony of complete resignation over the Polish crown was his attendance to the ceremonies of baptism, marriage and coronation of Jadwiga and Jogaila in Kraków. After these ceremonies, he renewed his homage to the royal couple. After this, he joined the royal entourage to Vilnius, where he participated in the process of Christianization of Lithuania.
After finally admitted his defeat and paid homage to the Polish King, the political situation of Siemowit IV was significantly deteriorated. Before the war, as an independent ruler he can effectively maneuver between the Polish, Lithuanians and the Teutonic Order; now, as a vassal was clearly seen as an ally of the Polish Kingdom. In addition, in order to finance his policies he needed money and several times he mortgaged some of his domains to the Teutonic Order, including Wizna (during 1382-1401), P?o?sk (during 1384-1399) and Zawkrze (during 1384-1399 and 1407-1411).
In view of the growing friction between the Polish and Teutonic Order, Siemowit IV tried to obtain the greatest benefit for him and intervened as a mediator. Also, after the outbreak of the war of 1409-1410 between Poland, Lithuania and the Teutonic Order, the attitude of the Masovian Duke wasn't clear: in one side, he tried to contact King Sigismund of Hungary and by other side, he pressured his warring neighbors to maintain the peace. In view of the failure of his attempts to make a compromise, Siemowit IV finally sent his troops at the Battle of Grunwald, but his participation was only symbolic; in fact, was his son Siemovit V who fielded two banners of his own troops and fought alongside the 'Royal' Poles and Lithuanian troops. In order to maintain his friendly relations with the Teutonic Order, he provides them with refunds even during the campaign; in exchange, the Order returned Zawkrze to Siemowit IV, despite the fact that under the Peace of Thorn (1411) they aren't obliged to do it.
Despite his official subordination to Poland, Siemowit IV tried to pursue an independent foreign policy. 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.
Siemowit IV's relations with Poland, although some temporary frictions caused by his too independent policy (he even minted his own coins) remained friendly, despite the fact that he didn't fulfill his duties as a vassal, and only sent troops to Poland occasionally when he was required to do. Another gesture of friendship with King W?adys?aw 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 domestic politics, Siemowit IV continued the economic restructuring which begun under the rule of his father. For this purpose, in addition to the existing statutes he implemented the Kulm law in several of his cities and promoted the colonization of the Masovian nobility to Belz.
After 1420 Siemowit IV, due to his progressive blindness, gradually gave participation in the government to his adult sons. In 1425, the dispute about the election of his Chancellor Stanis?aw z Paw?owic as Bishop of P?ock didn't brought anything good to Masovia, and only forced his sons Siemowit V and Casimir II into a humiliating surrender.
Siemowit IV died on 21 January 1426 at Gostynin and was buried in the Ducal crypt at P?ock Cathedral.
Prior to his marriage, Siemowit IV fathered an illegitimate son, Miklusz (also called Miko?aj; born before 1387), who was legitimated on 29 June 1417 by Emperor Sigismund. Nothing more is known about him.