Janusz I of Warsaw (pl: Janusz I warszawski), also known as Janusz I the Old (pl: Janusz I Starszy) (c. 1347/52 – 8 December 1429), was a Polish prince member of the House of Piast in the Masovian branch, from 1373/74 Duke of Warsaw and after the division of the paternal inheritance between him and his brother in 1381, ruler over Nur, Łomża, Liw, Ciechanów, Wyszogród and Zakroczym. In addition, he was a vassal of the Polish Kingdom since 1391 for the fief of Podlachia (only during his lifetime). He was the eldest son of Siemowit III, Duke of Masovia and his first wife Euphemia, daughter of Nicholas II of Opava. Due to an error of chronicler Jan Długosz was previously assumed that Janusz I was born c. 1329, and it wasn't until modern time that this date could be corrected until a much later one, c. 1346. Evidence of this fact was that only in 1373/74 he received his own duchy (with its capital in Warsaw). As the result of the partition of Masovia between him and his younger brother Siemowit IV after the death of their father on 16 June 1381, Janusz I finally obtain the totally of his domains: Warsaw, Nur, Łomża, Liw, Ciechanów, Wyszogród and Zakroczym. Policy with the Kingdom of Poland In this area, Janusz I faithfully maintained a close cooperation with the successive Polish rulers: Louis of Anjou, Jadwiga and Władysław II Jagiełło. One expression of this were the three homages performed by him in the years 1373, 1383 and 1387. In this way, Janusz I was directly opposed to the policy of his brother Siemowit IV, who tried to take advantage of the difficulties in the Angevin dynasty and wanted to obtain the Polish crown for himself. After the death of King Louis of Poland and Hungary, Janusz I recognized the rights of Jadwiga to the Polish crown. To this end, in 1383 he went to Buda, where he offered support forces, in return for which he received a salary of 24,000 florins per year taken from the salt mines of Bochnia. His pro-Angevin policies soon caused that Janusz would be protected from the army of the future Sigismund of Luxemburg, the future Holy Roman Emperor, in his way to rescue his future wife Mary, Queen of Hungary and Jadwiga's sister. Policy towards Lithuania and seizure of Podlachia The preference for the Polish