The Battle of the Day River (French: bataille du Day) took place between late May and early June 1951, around the Day River Delta in the Gulf of Tonkin. Part of the First Indochina War, the battle was the first conventional campaign of Vo Nguyen Giap, and saw his Viet Minh forces tackle the Catholic-dominated region of the Delta in order to break its resistance to Viet Minh infiltration. On the back of two defeats at similar ventures through March and April that year, Giap led three divisions in a pattern of guerrilla and diversion attacks on Ninh Binh, Nam Dinh, Phu Ly and Phat Diem beginning on May 28 which saw the destruction of commando François, a naval commando. The French army, under Jean de Lattre de Tassigny, who lost his son in the first day of the battle at Ninh Binh, mobilised three mobile groups (groupements mobiles, similar to regimental combat teams) and two paratrooper battalions as well as one dinassaut, and the ebb and flow of captured and retaken positions continued until Giap's supply lines were cut around June 6. His forces, moving in large numbers and during daylight, were vulnerable to French firepower and to French ground forces supported by friendly local militia. The Viet Minh were forced into withdrawing between June 10 and June 18, leaving 1,000 prisoners to the French and 9,000 casualties.