In 1787, Vũ Văn Nhậm led an army marched north into Tonkin.
Nhậm occupied Thăng Long (mordern Hanoi), had Nguyễn Hữu Chỉnh executed, installed Lê Duy Cận as a puppet "Prince Regent" (監國 giám quốc).
Sở and Lân regarded it as treason, and reported Nhậm's actions to Nguyễn Huệ.
Ngô Văn Sở decided to retreat, but was opposed by Lân.
Sở concealed the fact and retreated to Tam Điệp, and sent Nguyễn Văn Tuyết to Phú Xuân (mordern Huế) to ask support.