The Lithuanian Civil War of 1432-1438 was a conflict over the succession to the throne of the Grand Duchy of Lithuania, after Vytautas the Great died in 1430 without leaving an heir.  The war was fought on the one side by Švitrigaila, allied with the Teutonic Knights, and on the other by Sigismund Kęstutaitis, backed by the Kingdom of Poland. The war threatened to sever the Union of Krewo, the personal union between Poland and Lithuania. Švitrigaila's alliance with the Grand Master of the Teutonic Order, Paul von Rusdorf, launched the Polish-Teutonic War  but failed to secure victory for Švitrigaila. When Sigismund captured power in Lithuania by staging a coup in 1432, Lithuania split into two opposing camps, and there began three years of devastating hostilities. To prevent the Knights from continuing their support of Švitrigaila, Poland backed a Hussite invasion of Prussia in 1433. The war ended in a decisive defeat for Švitrigaila and his ally, the Livonian branch of the Teutonic Knights, at the Battle of Pabaiskas in September 1435. Švitrigaila eventually surrendered in 1437; Sigismund Kęstutaitis ruled Lithuania for only eight years before he was assassinated in 1440.
Question: What event threatened the Union of Krewo?
Answer: Lithuanian Civil War of 1432-1438