[David E. Hoffman is a contributing editor to the Washington Post and the author of The Dead Hand: The Untold Story of the Cold War Arms Race and Its Dangerous Legacy.]

Just one week into his presidency, on Jan. 27, 1969, Richard M. Nixon got an eye-opening briefing at the Pentagon on the nation's secret nuclear war plans -- the Single Integrated Operational Plan, as it was known then. "It didn't fill him with enthusiasm," Henry Kissinger, the national security advisor, said later. The briefers walked Nixon through the absolutely excruciating decision a president would face upon receiving an alert of impending attack: whether to launch nuclear missiles.

The slides used to brief Nixon that day have been partially declassified and released by the National Security Archive, and they suggest how complex the whole decision process would be.  In the event of nuclear war, Nixon was told, he would have three functional tasks: Alpha, for strikes on the most urgent military targets; Bravo, for secondary military targets; and Charlie, for industrial and urban targets. If the president ordered an attack of Alpha and Bravo, urban areas would be spared. But beyond these were dozens of decisions, attack options, targets, and variations. There were committed forces and coordinated forces, hard-core forces and theater forces. Nixon was shown the "decisions handbook" or black book, with tabs, which was open in front of him.

At the end of the briefing, Nixon was shown a slide marked "Conclusion." He was reassured the war plan was flexible and responsive. "Procedures for execution are straight-forward and in themselves neither new or unusually complicated," Nixon was told. "It is in the decision-making process, the evaluation and selection of the many attack responses available, wherein the problem becomes complex."

Then the briefer warned:

"In a crisis mounted over a period of time, it should be possible to eliminate early some of the alternatives, such as whether or not to attack particular countries. In a long, drawn out crisis, with highly intensified force readiness on one or both sides, it may be even possible to eliminate from further consideration some of the attack options. But in a sudden emergency, with little or no warning, all of these considerations must be entertained and discussed with the president [pause] and perhaps in no more than a very few minutes."

Such a nightmare scenario hung over the Cold War until the very end, and even beyond. No one really could predict how a president or Soviet leader would react when faced with a do-or-die choice in just minutes. The imponderables troubled every American and Soviet leader of the nuclear age. And the high state of readiness of the weapons, on alert to fire in a short period, reflected the very deep tensions of the era....