The good relationship with the Polish Kingdom was broken unexpectedly in 1432, when Siemowit V began talks with the Teutonic Order in Działdowo and with Švitrigaila in Vilnius for a joint alliance against Poland.
There is speculation that the contacts between Siemowit V and Švitrigaila were made with the knowledge and consent of King Władysław II, evidenced by the fact that after all these events the Masovian ruler maintained his proper relations with the Polish crown.
During this time, Siemowit V's bad relations with Teutonic Order caused his frequent contacts with the Duchy of Pomerania and his participation in the expedition of the Hussites in 1433 at Pomerelia.
After a meeting in Opatów on 13 July 1434, where the Bishop vigorously defended the rights of the late King's children, the candidacy of Siemowit V was abandoned.
In view of the failure of his plans to obtain the crown, Siemowit V quickly accepted the new ruler Władysław III and on July 25, with his brother Casimir II, attended the coronation of the young monarch.
[1] In April 1438 he participated in the Confederation of Nowy Korczyn, where the nobility discouraged King Władysław III to reject the Bohemian crown for his brother Prince Casimir.
Between 15 October 1434 and 17 February 1437,[2] Siemowit V married Margareta (1410 - 5 July 1459), a daughter of John II, Duke of Opava-Racibórz and widow of Casimir I of Oświęcim.