LeMay said, "Flying fighters is fun. Flying bombers is important". Aircraft losses on tactical attack missions soared, and Air Force commanders soon realized that their large, missile-armed jet fighters were exceedingly vulnerable not only to antiaircraft shells and missiles but also to cannon-armed, maneuverable Soviet fighters. LeMay advocated a sustained strategic bombing campaign against North Vietnamese cities, harbors, ports, shipping, and other strategic targets. His advice was ignored. Instead, an incremental policy was implemented that focused on limited interdiction bombing of fluid enemy supply corridors in Vietnam, Laos, and Cambodia. This limited campaign failed to destroy significant quantities of enemy war supplies or diminish enemy ambitions. Bombing limitations were imposed by President Lyndon Johnson for geopolitical reasons, as he surmised that bombing Soviet and Chinese ships in port and killing Soviet advisers would bring the Soviets and Chinese more directly into the war. In his 1965 autobiography (co-written with MacKinlay Kantor), LeMay is quoted as saying his response to North Vietnam would be to demand that "they've got to draw in their horns and stop their aggression, or we're going to bomb them back into the Stone Age. And we would shove them back into the Stone Age with Air power or Naval power—not with ground forces". LeMay subsequently rejected misquotes of the famous "Stone Age" quote. Later, in a "Washington Post" interview LeMay said that "I never said we should bomb them back to the Stone Age. I said we had the capability to do it. I want to save lives on both sides". Etymologyst Barry Popik cites multiple sources (including interviews with LeMay) for various versions of both quotes from LeMay Nevertheless, the "should" quote remained part of the LeMay legend, and remains widely attributed to him ever after. Some military historians have argued that LeMay's theories were eventually proven correct. Near the war's end in December 1972, President Richard Nixon ordered Operation Linebacker II, a high-intensity Air Force, Navy, and Marine Corps aerial bombing campaign, which included hundreds of B-52 bombers that struck previously untouched North Vietnamese strategic targets, including heavy populated areas in Hanoi and Haiphong. Linebacker II was followed by renewed negotiations that led to the Paris Peace Agreement, appearing to support the claim. However, consideration must be given to significant differences in terms of both military objectives and geopolitical realities between 1968 and 1972, including the impact of Nixon's recognition and exploitation of the Sino-Soviet split to gain a "free hand" in Vietnam and the shift of Communist opposition from an organic insurgency (the Viet Cong) to a conventional mechanized offensive that was by its nature more reliant on industrial output and traditional logistics.