built by Kęstutis Kęstutis wielded significant influence during the reign of Algirdas, and it appears the two were officially
co-rulers as Grand Dukes, with Algirdas being the dominant co-ruler. It is uncertain at what point Kęstutis started to use the title of the Grand Duke, but he is attested in the office in years 1349-1351. 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.
Algirdas willingly remained aside of the business and was concerned with the order in the Ruthenian part of the state. The intermediary in the negotiations, Polish King
Casimir III, made an unexpected assault on
Volhynia and
Brest in October 1349, which ruined Kęstutis' plan. During the
Polish-Lithuanian war for Volhynia, King
Louis I of Hungary made a peace agreement with Kęstutis on 15 August 1351, according to which Kęstutis obliged himself to accept
Christianity and provide the
Kingdom of Hungary with military aid, in exchange for the royal
crown. The agreement was approved with a pagan ritual by Kęstutis to convince the other side. In fact, Kęstutis had no intentions to comply with the agreement and ran away on the way to
Buda. ==In crusaders' captivity==