Following gaining independence, Swietopelk II began developing the administration hierarchy in his duchy, appointing castellans, voivodes, deputy chamberlains, treasurer and others.

[3][4] On 15 June 1236, Swietopelk announced to his brothers that he had designated his oldest son, Mestwin II, as his successor as the duke of Pomerelia.

Around 1237, the Mecklenburgian fleet began fighting with Gdańsk forces alongside the coasts of the Baltic Sea, later on entering the Vistula river.

In 1237 or at the beginning of 1238, Ratibor fortified Białogarda, and, assisted by troops of Kuyavian bishop, attacked the Schlawe and Stolp Land, which he subsequently conquered and plundered.

[5] In August 1238, Swietopelk organized the raid on the Duchy of Kuyavia, aimed against duke Casimir I and bishop Michał, as the response to the previous attack.

During the attack, his army destroyed around 23 villages and the properties of bishops and monasteries, as well as devastating the town of Inowrocław, and stole a massive number of farm animals.

[8][10][11] On 30 November 1238, Swietopek was put on trial by Polish episcopal conference, against bishops of Kuyavia and Płock, Strzelno monastery and Gdańsk Dominicans, all of whom were victims of his raid.