[citation needed] Kęstutis employed different military as well as diplomatic means in his struggle on the western borders of the Grand Duchy of Lithuania.

In 1349, to avoid further clashes with the Teutonic Order, he started negotiations with Pope Clement VI for the Christianization of Lithuania, receiving promises for royal crowns for him and his sons.

The intermediary in the negotiations, Polish King Casimir III, made an unexpected assault on Volhynia and Brest in October 1349 that ruined Kęstutis's plan.

A small squad of Teutonic knights which was returning from the failed , attacked the camp of Kęstutis and Algirdas near the Ublik (Wobel) lake.

He, with the help of his servant Alfas, broke a hole in a three meter-thick wall and left the castle by horses dressed up as Teutonic knights.

Kęstutis and Vytautas continued to recognize Jogaila's authority even when his right of inheritance was challenged by Andrei of Polotsk, Algirdas' eldest son from the first marriage with Maria of Vitebsk.

[6] The Teutonic Knights continued their crusade against pagan Lithuania and both Jogaila and Kęstutis looked for opportunities to establish a truce.

[11] Jogaila, having secured his western front, allied himself with the Golden Horde against the Grand Duchy of Moscow for the upcoming Battle of Kulikovo.

Jogaila was away to subdue the rebellion and his absence provided a good opportunity to capture Vilnius, capital of the Grand Duchy.

[6] Kęstutis resumed war with the Teutonic Knights: his army raided Warmia and attempted to capture Georgenburg (Jurbarkas).

[7] On 12 June 1382, while Kęstutis was away to fight Dymitr Korybut of Novhorod-Siversky and Vytautas was away in Trakai,[12] residents of Vilnius, led by merchant Hanul of Riga, let Jogaila's army into the city.

[7] In the meantime Kęstutis rallied his supporters in Samogitia, his son Vytautas sought soldiers in Hrodna, and his brother Liubartas recruited in Galicia–Volhynia.

[12] Jogaila organized a large pagan funeral to Kęstutis: his body was burned with horses, weapons, and other treasures in Vilnius,[12] presumably in Šventaragis' Valley.

Kęstutis is featured in the 2021 video game Age of Empires II: Definitive Edition - Dawn of the Dukes in a campaign detailing the exploits of himself and his brother Algirdas.