 In order to gain control of these vast holdings, Lithuania had fought the Battle of Blue Waters in 1362 or 1363 against the invading Mongols and had taken advantage of the power vacuum to the south and east that resulted from the Mongol destruction of Kievan Rus'.The population of the Grand Duchy's enlarged territory was accordingly heavily Ruthenian and Eastern Orthodox.The territorial expansion led to a confrontation between Lithuania and the Grand Duchy of Moscow, which found itself emerging from the Tatar rule and itself in a process of expansion.