WASHINGTON — The United States went on high alert in October 1973 because of intelligence indicating that the Soviet Union was delivering nuclear weapons to Egypt at the height of the Yom Kippur War, newly declassified CIA papers show.

It has long been known that the National Security Council under President Richard Nixon ordered a DEFCON III (or Defense Condition III) alert, the highest available in peacetime, because of a possible nuclear threat against Israel.

What was not known until last week, when the CIA declassified its briefings for Nixon, was what triggered the concern.

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Tim Naftali, a presidential historian who has directed the Nixon presidential library, reviewed the documents in a post Friday on CNN’s website.

“American intelligence had detected a Soviet ship headed for Egypt that it believed was carrying nuclear weapons,” Naftali wrote. “In addition, the United States detected two Soviet amphibious ships nearing Egypt.”

Previous reporting showed that the Soviets were alarmed by the DEFCON III alert and sought to quickly defuse the situation, reassuring the Americans that no nuclear action was planned.

The DEFCON III alert put in place on October 24, 1973, was removed the next day. The Soviet ships near Egypt dispersed, Naftali wrote. However, it is not known what happened to the ship that reached Egypt, which intelligence officials believed to be carrying nuclear weapons.