The battalion was also stricken by malaria during this time; 206 3rd Battalion Marines contracted it. In the fall the battalion was put to work building and defending the McNamara Line, a series of fixed fortifications along the DMZ in an area known as Leatherneck Square (The four corners were Cam Lộ, Con Thien, Gio Linh and Đông Hà). The PAVN repeatedly attacked the positions and twice 3rd Battalion had to come rescue elements of 2nd Battalion, 4th Marines that were in danger of being overrun. 3rd Battalion would occupy several of those positions through May 1968. From 29 December to 12 January elements of the battalion participated in the construction and defense of strongpoint Alpha 3. The first major combat for 3rd Battalion in 1968 came on 7 February, about a week after the Tet Offensive began, when Kilo Company engaged a PAVN battalion near Gio Linh. 29 Kilo Marines were killed, including PFC Robert Quick who was awarded a posthumous Navy Cross for deflecting a PAVN hand grenade. Another 31 were wounded, including PFC Craig Swartz who was wounded three times and received three purple hearts for his service. PAVN losses numbered at least 139, with another 60 graves discovered by 3rd Battalion Marines three days later. That same month a platoon from Mike Company observed and damaged two PAVN PT-76 tanks near Alpha 3, one of only three times they were encountered during the Vietnam War before 1972. On 6 March, Mike Company blundered into the 27th Regiment of the PAVN 325th Division and had one-third of the company put out of action in an all-night engagement, including 14 Marines killed. In early May the PAVN launched a new offensive known as the May Offensive or 'mini-Tet'. During that time, on 8 May, 3rd Battalion overran a PAVN regimental command post in Leatherneck Square, suffering 102 casualties including 8 killed. On 22 May, Lima Company uncovered a North Vietnamese staging area near Alpha 3 and destroyed it in an action that involved elements of three other Marine battalions. Then on 26 May the battalion was moved east to Dai Do as part of Operation Napoleon/Saline, in response to PAVN pressure on the supply lines to the 3rd Marine Division headquarters at Dong Ha. By the end of the month, 3rd Battalion had lost 53 Marines and had another 319 severely wounded, a casualty ratio of about one out of every two infantrymen in the field.