On May 11, 1972, the PAVN launched a massive all-out infantry and armor (T-54 medium tanks) assault on An Lộc. The attack was carried out by units of the 5th and 9th PAVN divisions. This attack was repulsed by a combination of U.S. airpower and the determined stand of ARVN soldiers on the ground. Almost every B-52 in Southeast Asia was called in to strike the massing enemy tanks and infantry. The commander of the defending forces had placed a grid around the town creating many "boxes", each measuring 1 km by 3 km in size, which were given a number and could be called by ground forces at any time. The B-52 cells (groups of three aircraft) were guided onto these boxes by ground-based radar. During May 11 and 12, the U.S. Air Force managed an "Arc Light" mission every 55 minutes for 30 hours straight—using 170 B-52s and smashing whole regiments of PAVN in the process. Despite this air support, the PAVN made gains, and were within a few hundred meters of the ARVN 5th Division command post. ARVN counter-attacks were able to stabilize the situation. By the night of May 11, the PAVN consolidated their gains. On May 12, they launched new attacks in an effort to take the city, but again failed. The PAVN launched one more attack on May 19 in honor of Ho Chi Minh's birthday. The defenders were not surprised, and the attack was broken up by U.S. air support and an ambush by the ARVN paratroopers.