Complex sentence: The Ming Chinese had helped the Oirat's rise to power over the Mongols during the Ming Yongle Emperor's reign after 1410 when the Ming defeated the Qubilaid Öljei Temür until the Ming and Oirats ended the alliance when Yongle launched a campaign against them.
Split into simpler sentences: The Ming Chinese had helped the Oirat's rise to power over the Mongols during the Ming Yongle Emperor's reign after 1410 when the Ming defeated the Qubilaid Öljei Temür. The Qubilaids were displaced from power by the Oirats with Ming help until the Ming and Oirats ended the alliance when the Yongle Emperor launched a campaign against them.