 Complete the following phrase:  Bach Mai Airfield was used as the air defense command and control center for the Vietnamese Peoples Air Force during the Second Indochina War. Due to its location in the restricted area of a radius of 30 nautical miles reduced to 10 in 1967 from the center of Hanoi and its proximity to the Bach Mai Hospital 1 km away, Bach Mai Airfield was off limits to US bombers during the early years of Operation Rolling Thunder with the result that North Vietnamese command and control was generally unmolested. Presidential approval was required for an attack on the airfield and it was attacked for the first time by F-105 Thunderchiefs of the 388th Fighter Wing on 17 November 1967. Bombing restrictions were lifted during Operation Linebacker I and the airfield was attacked on 16 May 1972. On 21 December 1972 in 