On 31 October 1968 President Johnson, under growing pressure from public opinion at home and no signs that the North Vietnamese were about to back down, announced that the bombing of North Vietnam would cease the following day. The Rolling Thunder bombing campaign against North Vietnam had started on 2 March 1965 but during its 44-month history its overall effectiveness was severely restricted by political limitations and the resourcefulness of the North Vietnamese. So began a three-and-a-half year bombing halt against North Vietnam that would last until April 1972. In the meantime reconnaissance missions could still be flown and these were often escorted by F-4s or F-105s. The defences in North Vietnam at the time of the bombing halt were formidable and consisted of 8,000 anti-aircraft guns, 400 radars, 150 MiGs and 40 SA-2 sites. During the halt the North Vietnamese took the opportunity to reinforce their air defences in all areas in case the Americans should resume their bombing.