The Siege of Ba Đình (December 1886 to January 1887) in Thanh Hóa province was a decisive engagement between the insurgents and the French. The siege was deliberately willed by the Vietnamese resistance leader Đinh Công Tráng, who built an enormous fortified camp near the Tonkin-Annam border, crammed it full of Annamese and Tonkinese insurgents, and dared the French to attack him there. The French obliged, and after a two-month siege in which the defenders were exposed to relentless bombardment by French artillery, the surviving insurgents were forced to break out of Ba Đình on 20 January 1887.