Using B-52's for “close-in” bombing was the result of an error made several months before thatrevealed a previously unknown vulnerability.
A routine B-52 mission flown from U Tapao, Thailand, on 12 November 1967 contributed quiteby accident to an important tactical innovation. Nine B-52's took off from U Tapao to hit troopconcentrations and rocket batteries in the vicinity of Con Thien, but one of the planes failed toobserve the 3-kilometer safety zone established to keep bombs from falling accidentally amongfriendly troops. This particular plane—there is doubt as to which one—dropped its explosiveswithin the safety zone about 1.4 kilometers from Marine lines.
Neither the men defending Con Thien nor their fortifications suffered harm from this error.Indeed, the results from the misdirected bomb load verged on the astonishing, as secondaryexplosions blossomed near the defensive perimeter. The enemy was clearly taking advantage ofthe safety zone imposed on the Stratofortresses, a fact that lent greater urgency to an ideadiscussed the previous summer, the use of B-52's in what amounted to close air support.
Although it was clear that the enemy was aware of the 3 kilometer limit and was using this informationto his advantage, the change of procedures to reduce safety zones did not come without discussion. Tendays after the revelation at Con Thien, General Gillem contacted Lt. Col. McGowan, Commander ofDetachment 15 at Tan Son Nhut, about the possibility of implementing the new procedures. Lt. Col.McGowan wrote on November 22, 1967:
Gen Gillem at SAC Headquarters sent a message to me asking my opinion on a change inprocedures, and he also sent a message to Col Hutchison [CEG Commander] asking his opinion.This morning a copy of Col Hutchison's opinion was received. He told Gen Gillem that he wasviolently opposed to use of these procedures. My reply to Gen Gillem was that I was extremelyin favor of it and it should be implemented immediately, if not sooner. Gen Gillem bought myrecommendation and the procedures were put into effect.
Col. Hutchison's comments must have had an effect, because the very next day on November 23, Gen.Gillem reversed course and the procedures were not adopted.
His [Col. Hutchison] message to Gen Gillem at SAC scared Gen Gillem into canceling hisdecision to accept our procedures. Lost the first round but I'm sure I can convince Gen. Gillem. 
It was only two months later when the battle of Khe Sanh presented the opportunity, and perhaps the necessity, to allow bombing perilously close to friendly troops.
Discussions began in January of 1968, with recommendations that close-in bombing only be used in emergencies. General Westmoreland requested Lt. Gen. Cushman, commander of the III Marine Amphibious Force, to request strikes within the 3 kilometer zone after being “dissatisfied” with B-52 effectiveness. Admiral Sharp's headquarters reviewed the matter and approved B-52 strikes as close as 1,000 meters to friendly troops “in emergencies”.
 Nalty, The Fight for Khe Sanh, 83
 Lt. Col. F. D. McGowan, Commander, Det. 15, Tan Son Nhut, Private Correspondence, 22 November 1967
 Lt. Col. F. D. McGowan, Commander, Det. 15, Tan Son Nhut, Private Correspondence, 23 November 1967