In late March 1972, the North Vietnamese launched a full-scale offensive across the Vietnamese Demilitarized Zone, supported by tanks and heavy artillery. By this time, the US was no longer in the forefront of the ground war, with South Vietnamese units taking the lead. However, the US was still providing air power, and President Richard Nixon ordered a large increase on US airpower in response to the invasion. Although there had been no campaign of strikes into North Vietnam since the end of Rolling Thunder, the Nixon Administration ordered a new air offensive, initially code named Freedom Train, later becoming Operation Linebacker, with relatively few restrictions on targets that could be hit.