On January 13, 1951, Giáp moved the 308th and 312th Divisions, made up of over 20,000 men, to attack Vĩnh Yên, 20 miles (32 km) northwest of Hanoi, which was manned by the 6,000-strong 9th Foreign Legion Brigade.
In late January, upon finding that the PVA had abandoned their battle lines, General Ridgway ordered a reconnaissance-in-force, which became Operation Thunderbolt (25 January 1951).