At first he was defeated by Chinese army, later, he was supported by two Vietnamese generals, Đặng Tất and Nguyễn Cảnh Chân, and occupied Nghệ An Province successfully.
Giản Định decided to attack Đông Đô (modern Hanoi) but was opposed by Đặng Tất and Nguyễn Cảnh Chân.
Giản Định soon had both Đặng Tất and Nguyễn Cảnh Chân arrested and killed, causing dissent and revolt in his army.
Giản Định was arrested by Trùng Quang in 1409, and was granted the title thái thượng hoàng ("Retired Emperor").
He was sent to Hạ Hồng (modern Ninh Giang District, Hải Dương Province) to attack Chinese army, but was defeated by Zhang Fu and fled to Diễn Châu.