He said that while the document conjured the height of the Cold War, the targeting of urban populations still remained an underlying principle of the use of nuclear weapons to deter attack. “The heart of deterrence is the threat to destroy the adversary’s cities, even today,” Mr. McKinzie said.

Alex Wellerstein, a historian of nuclear weapons at the Stevens Institute of Technology in Hoboken, said that in 1959, the United States had atomic bombs totaling about 20,000 megatons. President Dwight D. Eisenhower pushed to reduce the arsenal, and the tonnage was cut by half over the next year or two, he said.

“He just thought this would lead to the annihilation of the human species,” Mr. Wellerstein said.

At the time, military planners sought to surround the Soviet Union with bomber bases and, in the event of war, called for what they referred to in official documents as a “bomb as you go” strategy, flying toward the biggest Soviet cities and hitting every listed target along the way, Mr. Wellerstein said.

The 1956 document makes air power the highest-priority target, including 1,100 Soviet-bloc airfields, since the goal was to destroy Soviet bombers before they could take off and head for targets in Europe and beyond. But many air bases and command centers were in and around population centers, so even those strikes would have resulted in extensive civilian casualties.

The targets with the second-highest priority were those of the industrial infrastructure. That included the people who ran it.

Several military historians said Tuesday that while the general principle that civilians should not be targeted dated to before World War I, actual practice had often been dictated by the military needs of the moment. The allies in World War II and the Korean War began with a principle of avoiding killing civilians to the extent possible. But in each conflict, that ideal often gave way to bombing cities because it was seen as a military necessity.

Targeting civilians has often been viewed as a way of undermining enemy morale, prompting a revolt or surrender — and conceivably leading to a shorter war. And so the large-scale bombing of civilians has sometimes been defended on humanitarian grounds, even after the firebombings of Tokyo and Dresden, Germany, and the atomic bombs dropped over Hiroshima and Nagasaki.