Khalkha Mongols
The Four Oirat was a political entity formed by the four major Oirat tribes. During the 15–17th centuries, they established under the name "10 tumen Mongols", a cavalry unit of 10,000 horsemen, including four Oirat tumen and six tumen composed of other Mongols. They reestablished their traditional pastoral nomadic lifestyle during the end of the Yuan dynasty. The Oirats formed this alliance to defend themselves against the Khalkha Mongols and to pursue the greater objective of reunifying Mongolia.