The Grand Duchy of Lithuania to which Jogaila succeeded as Grand Duke in 1377 was a political entity composed of two leading, but very different nationalities and two political systems: ethnic Lithuania in the north-west and the vast Ruthenian territories of former Kievan Rus', comprising the lands of modern Ukraine, Belarus, and parts of western Russia. At first, Jogaila—like his father—based his rule in the southern and eastern territories of Lithuania, while his uncle, Kęstutis, the Duke of Trakai, continued to rule the north-western region. Jogaila's succession, however, soon placed this system of dual rule under strain.