But the United States government finally released an official copy to the National Security Archive and in a new State Department history of the era published last month.

In a 1989 oral history, Mr. Blood kept quiet about the possible change in the relationship. Instead, he focused on an episode earlier in the year when the United States ambassador to Afghanistan, Adolph “Spike” Dubs, was kidnapped in Kabul and killed during a rescue attempt.

“Washington asked me to seek an appointment with Hafizullah Amin who was the president and the leader,” Mr. Blood, who died in 2004, said in the oral history. “About the only thing they wanted to tell him was that he couldn’t expect any resumption of aid until he could satisfy us about their role in Spike’s death.”

Rodric Braithwaite, the last British ambassador to the Soviet Union and the author of “Afgantsy: The Russians in Afghanistan, 1979-89,” said on Monday that it had long been known that the Kremlin worried that Mr. Amin was turning to the United States, but said Soviet leaders had multiple motives for the invasion.

“It’s difficult to weight all the considerations,” he said, “but the Russians’ main concern was to ensure that a country on their vulnerable southern border, which they had cultivated for decades, didn’t become hostile.”

The Kremlin was also angered that Mr. Amin had not only toppled President Noor Muhammad Taraki, who had its backing, but had him killed. On Dec. 12, 1979, the Politburo approved a miliary intervention with no debate as Mr. Brezhnev and the others signed a handwritten decision memo titled “On the Situation in ‘A.’”

The Soviets tried to kill Mr. Amin only to botch it. The day after the “A” memo was signed, a K.G.B. operative slipped poison into his Coca-Cola, but the carbonation diluted the toxic agent. A couple of weeks later, the K.G.B. poisoned his food, but the Soviet Embassy in Afghanistan, unaware of the plot, sent doctors to save him. Only when thousands of Soviet troops poured into Afghanistan did they finally dispatch the troublesome leader, this time during crossfire.

The invasion was intended to be a quick operation, as in Hungary in 1956 and Czechoslovakia in 1968. But resistance to the Soviets was fierce and unrelenting. The realignment Mr. Blood broached took place as a result, with the United States coming to the aid of the Afghan rebels. It was, however, a realignment that would not last.