Task: Question Generation
Input (English): Passage: Švitrigaila was losing his influence in the Slavic principalities and could no longer resist Poland and Sigismund. On 4 September 1437 he attempted to reconcile with Poland: he would rule the lands that still backed him , and after his death these territories would pass to the King of Poland. However, under strong protest from Sigismund, the Polish Senate declined to ratify the treaty . In 1438 Švitrigaila withdrew to Moldavia. The reign of Sigismund Kęstutaitis was brief — he was assassinated in 1440. Švitrigaila returned from exile in 1442 and ruled Lutsk until his death a decade later. Jogaila's son Casimir IV Jagiellon, born in 1426, received approval as a hereditary hospodar from Lithuania's ruling families in 1440. This event is seen by the historians Jerzy Lukowski and Hubert Zawadzki as marking the end of the succession dispute.
Output (English): What caused Sigismund Kęstutaiti's reign to end?