own to regain ground ahead of the ceasefire. The US continued to support the South Vietnamese, principally with air strikes, until the ceasefire came into effect. Battle I Corps By late October 1972 the ARVN had recaptured Quảng Trị and held a line along the Thạch Hãn River. An attempt to advance to the south bank of the Cam Lộ/Cửa Việt River was repulsed in November. On 15 January 1973 the South Vietnamese Joint General Staff began planning began for a final assault by South Vietnamese Marines and ARVN to capture the south bank of the Cam Lộ/Cửa Việt River and the Cửa Việt Base. The attack was launched on the morning of 26 January and by 07:00 on 28 January the South Vietnamese had recaptured the base. At 08:00 Indochina Time on 28 January the ceasefire came into effect and US support for the operation ceased. On the evening of 29 January the PAVN counterattacked and by 31 January had recaptured the base. South Vietnamese losses were 40 killed and they claim to have inflicted over 1000 PAVN casualties. South and west of Quảng Trị the PAVN B5 Front forces prevented any expansion of the ARVN Airborne Division's positions into the hills south of the Thạch Hãn River and against the Thạch Hãn River line itself. The PAVN built up their defenses in the highlands north of the Thạch Hãn River and west of Quảng Trị. Elements of the PAVN 304th Division were shifted to this sector, and additional antiaircraft units were brought into the B5 Front, so that by the end of January elements of at least 11 antiaircraft regiments were deployed in northwestern Quảng Trị Province. In late January 1973 elements of the PAVN 263rd SAM Regiment, equipped with SA-2 missiles, deployed to the former Khe Sanh Combat Base. The regiment had four firing battalions, each equipped with four to six launchers and one support battalion. The regiment was apparently deployed to protect the PAVN logistics hub of the Ho Chi Minh Trail, the A Sầu Valley and Route 9. Further south in Thừa Thiên Province, on 24 January elements of the 803rd Regiment, 324B Division moved into the lowlands south of Camp Evans and north of Huế. On 25 January artillery and ground attacks began against ARVN positions around Huế. Between 27 January and 3 February elements of the 803rd Regiment attempted to interdict Highway 1