Angered that his Chinese and Vietnamese allies had done little to support the Black Flag Army at Son Tay, Liu stood on the sidelines during the Bắc Ninh Campaign (March 1884). After the French capture of Bắc Ninh, Liu retreated with the Black Flag Army to Hưng Hóa. In April 1884 the French advanced on Hưng Hóa with both brigades of the Tonkin Expeditionary Corps. The Black Flags had thrown up an impressive series of fortifications around the town, but General Charles-Théodore Millot, the French commander-in-chief, took it without a single French casualty. While General François de Négrier's 2nd Brigade pinned the Black Flags frontally and subjected Hung Hoa to a ferocious artillery bombardment from the Trung Xa heights, General Louis Brière de l'Isle's 1st Brigade made a flank march to the west to cut Liu's line of retreat. On the evening of 11 April, seeing Brière de l'Isle's Turcos and marine infantry emerging behind their flank at Xuan Dong, the Black Flags evacuated Hưng Hóa before they were trapped inside it. They set alight the remaining buildings before they left, and on the following morning the French found the town completely abandoned.