Around 570 AD, Lý Phật Tử's army overran Long Biên, a region that belonged to Triệu Việt Vương, and seized it.
Triệu Việt Vương was caught by surprise and defeated, he retreated to the river Đại Nha (now in Nam Định Province, northern Vietnam) where he killed himself.
Lý Phật Tử realized his army would not be strong enough to engage in a major conflict with the Sui invading force.
In the winter of 602, when the Sui force marched on Vạn Xuân, Lý Phật Tử decided to surrender in exchange for stability in the region.
When the northern imperial power began showing signs of weakness and disunity at the end of the Tang dynasty, the Việt people would seize the opportunity to slip from Chinese rule.