Complex sentence: The brothers forged alliances with King Władysław I of Poland and the Teutonic Knights against the Lithuanians and the Mongols, but the Kingdom was still tributary to the Mongols and joined the Mongol military expeditions of Uzbek Khan and his successor, Janibek Khan.
Split into simpler sentences: The brothers forged alliances with King Władysław I of Poland and with the Teutonic Knights against the Lithuanians and the Mongols. But the Kingdom was still tributary to the Mongols and joined the Mongol military expeditions of Uzbek Khan and his successor, Janibek Khan.