In Indochina, as in Korea, American soldiers and their allies, often without realizing it, were engaged in direct combat with Soviet and Chinese troops. On occasion the soldiers of Brezhnev and Mao were killing Americans in Vietnam. The decision of the Johnson administration not to invade North Vietnam, then, was based on a correct evaluation of direct Chinese military intervention, as well as on a well-informed fear of provoking a Soviet-American confrontation. The argument of the praetorian school that Johnson could have brought the war to a quick end by invading North Vietnam has been completely discredited.
Michael Lind, in Vietnam: The Necessary War (1999)