Casimir I received Kujawy from his father in 1233. In 1239, he would enlarge his domains with the castellany of Ląd, which he received as the dowry of his second wife. In subsequent years, Casimir I actively supported his father's turbulent politics, which brought him in 1242 to conquer the district of Wyszogród from the rulers of Gdańsk. Konrad I died on 31 August 1247. According to his will, most of Masovia passed to his oldest son, Bolesław I, and the youngest of his sons, Siemowit I, received Sieradz-Łęczyca. Dissatisfied with his part of the paternal inheritance, Casimir I attacked his brothers' lands while they were in Płock at the funeral of their father, and was able to conquer Siemowit I's inheritance. The next geopolitical change in Masovia came in early 1248, when Bolesław I died unexpectedly and without issue. In his will, he left all his domains to his brother Siemowit I. With this opportunity, Casimir I was able to take advantage of the confusion after his older brother's death and capture Dobrzyń. During the 1250s Casimir I was busy with his effort to Christianize the Yotvingian tribes and establish peaceful relations with them. His plans were not supported by the Teutonic Order, however, which had the Pope on its side. After this failure, Casimir I appealed to the Knights Templar, who settled in Łuków, in order to secure the northern border of his territory. It was not until 1263 that his relations with the Teutonic Order where normalized. Meanwhile, Casimir I faced additional internal problems. In 1258, Bolesław the Pious, Duke of Greater Poland, allied with Wartislaw III, Duke of Pomerania, and attacked Kujawy, demanding the return of the castellany of Ląd. It was his opinion that the land was unlawfully transferred by Henry II the Pious to his son-in-law. The expedition was only partially successful, and so the following year Bolesław the Pious organized a more powerful coalition with Bolesław V the Chaste, Siemowit I, and Daniel of Halych against Casimir I. This expedition was a complete victory, and on 29 November 1259, Casimir I was forced to surrender and promised to cede Ląd to Bolesław the Pious. Ultimately, however, Casimir I refused to return Ląd, and this caused the coalition to attack again in 1261. Leszek II the Black, Casimir I's eldest son, took advantage of his father's difficult position and demanded his own district, using as an excuse the intrigues of his stepmother, who wanted to obtain more land for her own children. In the end, Casimir I was forced to not only surrender the castellany of Ląd, but to also give Sieradz to his son as a separate district. Casimir died on 14 December 1267 and was buried in the Cathedral of Włocławek.
Marriages and Issue
Around 1230/31 Casimir I married firstly with Hedwig, probably the daughter of Władysław Odonic. They had no children. In 1239 Casimir I married secondly with Constance, a daughter of Henry II the Pious. As her dowry, she received the castellany of Ląd, which was the cause of the Casimir I's long conflict with the Duchy of Greater Poland. They had two sons:
Leszek II the Black.
Ziemomysł.
In 1257, Casimir I married thirdly with Euphrosyne, daughter of Casimir I of Opole. As her dowry, she received the district of Ruda, but the quick action of Przemysł I, Duke of Greater Poland, prevented this land from passing to the Masovian branch. They had four children:
