For two weeks in October 1969, the Nixon Administration secretly placed U.S. nuclear forces on alert. At the time, the move was considered so sensitive that not even the Chairman of the Joint Chiefs of Staff was briefed on its purpose. Still today, no conclusive explanation for the potentially destabilizing alert can be found. Even with full access to the classified record, State Department historians said in a new volume of the Foreign Relations of the United States (FRUS) series that they were unable to provide a definitive account of the event.

Previous historical scholarship has inferred from selected declassified documents that the alert was somehow intended to communicate a firm resolve to end the Vietnam War by whatever means necessary. (See “Nixon’s Nuclear Ploy” by William Burr and Jeffrey Kimball, National Security Archive, December 23, 2002; and “The Madman Nuclear Alert” by Scott D. Sagan and Jeremi Suri, International Security, Spring 2003.)

But based on the classified record, that interpretation remains unproven and uncertain, according to the gripping new State Department FRUS volume on “National Security Policy” (pdf).

“The documentary record offers no definitive explanation as to why U.S. forces went on this alert, also known as the Joint Chiefs of Staff (JCS) Readiness Test,” the editors of the FRUS volume said (Document 59).

“There are two main after-the-fact explanations: first, that nuclear brinkmanship was designed to convince the Soviets that President Nixon was prepared to launch a nuclear attack against North Vietnam in order to convince Moscow to put pressure on Hanoi to negotiate an end to the war in Southeast Asia” along the lines that previous historians have suggested.

The second proposed explanation is “that the President ordered the alert as a signal to deter a possible Soviet nuclear strike against China during the escalating Sino-Soviet border dispute.” Consistent with the second interpretation, the FRUS volume provides new documentation of intelligence reports indicating that Soviet leaders were considering a preemptive strike against Chinese nuclear facilities.

Astonishingly, even the most senior U.S. military leaders were kept in the dark by the White House about the nature of the alert– before, during and after the event.

“It is difficult to measure the success of this operation,” wrote JCS Chairman General Earle G. Wheeler to Secretary of Defense Melvin Laird on November 6, 1969, “since… the objectives of the test are unknown.”

“It seems prudent if maximum benefit is to be gained from an operation of this type that at least you and I and the senior commanders are informed of the objectives and goals,” General Wheeler suggested (Document 92).

In the end, the secret U.S. military alert — one of only a few such cases involving U.S. nuclear forces — had little discernable impact. “There has been no reflection of acute concern by the Soviets…,” the CIA reported in an October 27, 1969 memorandum included in the FRUS volume (Document 89). “There has been no reflection of the US military alert posture in Soviet or Chinese news media or diplomatic activity.”

Of the small White House group that directed the secret 1969 alert, perhaps only Henry Kissinger remains alive and active. He did not mention the alert in his memoirs, the FRUS editors noted, except perhaps in an oblique statement that the United States “raised our profile somewhat to make clear that we were not indifferent” to Soviet threats against Chinese facilities.