Smoke rises more than 60,000 feet into the air over Nagasaki from an atomic bomb, the second ever used in warfare, dropped from a B-29 Superfortress bomber in this U.S. Air Force handout photo dated August 9, 1945. Reuters "My God, what have we done," Enola Gay copilot Robert Lewis reportedly said as the Superfortress was making its way back to base on August 6, 1945, after dropping an atomic bomb on the Japanese city of Hiroshima.

World War II had ended with grisly evidence of what atomic warfare actually was. But just weeks after its end, the Pentagon was already hypothesizing about what an entire wave of nuclear attacks could achieve.

American defense planners came up with the following "estimated bomb requirements for the destruction of Russian strategic areas" in September of 1945:

In September 1945, US planners decided it would take 204 atomic bombs to destroy the Soviet Union. pic.twitter.com/6IhxDevZOu — OperationDesertPeace (@Nytzschy) October 3, 2014

The estimated amount of firepower needed to destroy each target city ranges from one to six atomic bombs, depending on the target's size. Sixty-six cities were identified in total, and it was determined that it would take 204 atomic bombs to put them all out of commission.

Historian and author Paul Ham cites similar numbers in his book Hiroshima Nagasaki. A US Army Air Force study found that the bombs would "obliterate most of Russia's population and industry — chiefly its capacity to refine oil and produce aircraft and tanks."

Alex Wellerstein, who blogs about the history of nuclear weapons and secrecy, explained some of the context of this 1945 assessment. He wrote that the Pentagon's finding actually called for more than 204 atomic weapons, for the simple reason that many of the bombs sent in this scenario would fail to destroy their targets.

"They assume, based on World War II figures, that a certain number of the bombers will get shot down, have technical problems, miss the target, or simply drop duds," Wellerstein wrote. "So they calculate that all of those bombs will only be 48% effective anyway, and thus they’ll need just over double the total number. So instead of about three bombs per city, they’ve allocated six."

In other words, it wasn't that 204 atomic bombs were deemed necessary to wipe out the Soviet Union. The US needed 204 atomic explosions (see the document above).

Below is a contemporary map of the intended targets.

The US probably wasn't planning a nuclear sneak-attack, and the 1945 study seem like more of a worst-case contingency than an operational plan. But the Cold War was clearly already underway mere weeks after World War II ended. The conclusion that America and Russia would be the world's leading military powers led to a fearful mentality even in a time before the Soviet Union was known to have nuclear arms.

In Deputy Chief of Air Staff Lauris Norstad's own phrasing, according to Ham's book, the salvo of 204 atomic bombs might be needed "to insure our national security."