How the U.S. Found Out About Russia’s First Nuclear Test 70 Years Ago

Today, detecting a nuclear test is as routine as sending someone into space.

But that wasn’t the case on Aug. 29, 1949, when the Soviet Union conducted its first nuclear test at Semipalatinsk, in northeastern Kazakhstan, shifting the nuclear arms race into high gear.

Less than two weeks after the test, U.S. intelligence knew something strange had happened—they just couldn’t say for sure what it was. As part of our Document of the Week series, Foreign Policy is highlighting a confidential Sept. 9, 1949, report by the then-CIA chief, Adm. Roscoe Hillenkoetter, that provided President Harry S. Truman with the first signs that the Soviets had succeeded in testing a nuclear bomb.

The Sept. 9 report—which was first published last month by the National Security Archive—cited the discovery of “an abnormal radio-active contamination” in the North Pacific. It is the latest in a series of documents the Washington-based archive has published over the past decade.

Pilots from the secret Air Force Office of Atomic Energy had collected traces of radiation in concentrations not seen since the United States had conducted a nuclear test in Enewetak on Bikini Atoll in August 1948. The radiation could have come from “An atomic explosion on the continent of Asia,” according to Hillenkoetter.

But it was also possible the radiation could have been produced by volcanic activity in the North Japanese Island, where a volcano had erupted on Sept. 2; radioactive gas from a nuclear plant in Hanford, Washington; or waste or an explosion at an atomic bomb plant in Russia.

“[T]here has been no confirmation of any man-made atomic explosion at this time,” Hillenkoetter wrote. “Meanwhile, we are attempting to get confirmation on exactly what might have occurred.”

It would be another 12 days until, on Sept. 21, U.S. intelligence confirmed to Truman that the Soviets, led by Joseph Stalin, had conducted their first atomic test, which they nicknamed “Joe 1.” Two days later, Truman publicly announced that the Soviets had formally demonstrated their capacity to test a nuclear bomb. The test had taken U.S. intelligence analysts, who predicted it could take the Soviets another five years to test an atomic explosive, by surprise. But the Soviets were already taken aback by the Americans’ ability to detect the test.