In autumn 1248, Siemowit I, together with Daniel, his brother Vasilko and Bolesław V the Chaste (whose domains were also affected by the Baltic invasions), organized the first major military campaign against the Yotvingians.

In the autumn of 1248, together with the Ruthenian army and the reinforcements of Prince Bolesław the Chaste, Siemowit took part in the armed conflict against Yotvingians.

Despite the devastation of large areas and the capture of many prisoners, the most important goal of the expedition, which was to subjugate this part of Yotvingia, was not achieved.

At the turn of 1255/1256, Siemowit I participated in a great expedition against the Yotvingians with the Ruthenian princes, as well as with the Lesser Poland knights from the principality of Bolesław the Chaste.

Another problem that plagued Siemowit I's reign was the relationship with his brother Casimir I, who feared (perhaps not without reason) that the alliance with the Teutonic Knights might also be directed against him.

Siemowit I and Pereyaslava were released the following year, following the intervention of Bolesław V the Chaste and others who promised to support Casimir I in his war against Swietopelk II, Duke of Pomerelia-Gdańsk.

The extremely aggressive policy pursued by Casimir I turned sour in 1259, when Bolesław the Pious, ally of Pomerania, set up a coalition against him with Boleslaw V the Shy, Siemowit I and Daniel of Halych.

Siemowit I and his oldest son, Konrad II, who were in Jazdów on a visit, prepared for a long siege of the fortress (especially when they expected that soon their allies could came to rescue them), however, due to the betrayal of the townspeople, the troops managed to enter Jazdów (23 June 1262); in the confusion of the battle, Siemowit I was killed and his son was captured; according to one version, the Masovian Duke was executed by his own brother-in-law Shvarn.

[2] During the captivity of Konrad II (which lasted two years), the regency of Siemowit I's domains was held by his ally Bolesław the Pious.