"For two weeks in October 1969, the Nixon Administration secretly placed U.S. nuclear forces on alert. [...] Still today, no conclusive explanation for the potentially destabilizing alert can be found.
“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."