By Paul Reynolds
World affairs correspondent, BBC News website
The Hungarian uprising in 1956 was a vital moment in the Cold War, showing both the aspirations of the peoples of Eastern Europe but also the determination of the Soviet Union not to lose its grip.
It also demonstrated the limits of Western power. Despite a desire to "roll back" the Soviet Empire in Europe, President Dwight D. Eisenhower did not help the Hungarians, in order to avoid the risk of general war.
Soviet tanks in Budapest after the uprising was crushed
And it coincided with another international crisis, Suez, the effect of which on Soviet actions has always intrigued historians.
However, secret documents that have emerged since the end of the Cold War also demonstrate that the Soviet intervention was not quite the cut-and-dried decision that it appeared at the time.
There was a brief moment when it hesitated.
It did so between the two phases of the uprising.
Phase one began on 23 October 1956 with a march by students to parliament by the Danube.
Their 16-point list of demands began with the most delicate of all - a call for the "immediate evacuation of Soviet troops".
The demonstration turned into a street battle when the secret police the AVH fired on the crowd. A statue of Stalin was pulled down, the Hungarian communist party called for Soviet help and Soviet tanks entered Budapest early the next day.
A statue of Josef Stalin was pulled down during an early demonstration
However, one of the students' demands - the return of a reformist former prime minister Imre Nagy - was conceded and the protest became a revolution.
Mr Nagy's new government drew up its demands. One of these, probably a fatal one, suggested that Hungary might leave the Warsaw Pact, the military agreement under which Soviet troops were stationed in Eastern Europe.
After fierce fighting, Soviet troops actually withdrew by 30 October to garrisons outside Budapest, while Moscow debated its next move.
So ended phase one.
Then the Soviet Union nearly blinked.
Minutes, or at least notes, taken at meetings of the Politburo in Moscow show the Soviet leader Nikita Khrushchev and his colleagues divided and uncertain during this period.
They hoped at first to resolve Hungary as they had just resolved a similar crisis in Poland. The Poles escaped intervention because they promised not to leave the Warsaw Pact.
On 23 October, Anastas Mikoyan, a Khrushchev ally, wanted a "Polish scenario".
On 28 October, Khrushchev was ambivalent. He appeared to side with the 'liberal' Mikoyan faction. Prime Minister Nikolai Bulganin spoke of intervention being "a dubious venture."
Khrushchev then raised another issue, the Suez crisis.
It is important to remember that Suez came to a head at exactly the same time as Hungary. Many thought it gave Moscow the cover to act.
The Israelis attacked across the Sinai desert on 29 October and the British and French bombing of Egypt began on 31 October.
At the 28 October meeting, Khrushchev appeared to be drawing the lesson that Suez was an adventure and that the Soviet Union should avoid a similar one in Hungary.
"The English and French are in a real mess in Egypt," he said. "We shouldn't get caught in the same company."
At a meeting on 30 October, the 'liberal' faction seemed to gain ground.
They might have been encouraged by a speech by the US Secretary of State John Foster Dulles on 27 October who declared that the Eastern European nations would not become "potential military allies."
Even Georgy Zhukov, the hero general who had captured Berlin, said: "We should withdraw troops from Budapest and if necessary from Hungary as a whole."
The result was an extraordinary statement from Moscow on 30 October. This declared that the Soviet Union "has ordered its military command to withdraw troops from Budapest. At the same time the Soviet government is ready to begin negotiations ...on the question of Soviet troops on Hungarian territory."
The statement took everyone by surprise.
However, a nastier surprise lay in store.
On the very next day, 31 October, the Politburo changed its mind. Discussion was dominated by Khrushchev, who said later that he had just spent a "lonely, sleepless night".
He told the Politburo: "We should take the initiative in restoring order in Hungary."
Khrushchev mentioned Suez again, this time less as an example to be avoided and more as an example to follow.
"If we depart from Hungary, it will give a great boost to the imperialists," he said. "They will perceive it as weakness on our part. To Egypt they will then add Hungary. We have no other choice."
Phase two was next and final.
On 4 November the Red Army descended on Budapest again. It was all over by 10 November.
The pattern for Soviet control was set. Czechoslovakia found that out in 1968. It was only in the fringe countries Romania and Albania that some latitude was given.
Yugoslavia, which was not in the Warsaw Pact, was also left alone. At the height of the Hungary crisis, the Politburo minutes note: "We should negotiate with Tito." The Soviet grip remained on the rest.
Sir Rodric Braithwaite, former British ambassador to Moscow, author of "Moscow 1941" and now writing about the Soviet intervention in Afghanistan, said: "The Hungarians were injudicious, the Russians were incompetent and the Americans could not deliver."
"Had Imre Nagy been loyal to the Warsaw Pact, the Russians might not have intervened. The same thing happened again in Poland in 1980. The Soviets nearly invaded but the Polish leader General Jaruzelski declared martial law himself instead.
"Once Nagy had mentioned leaving the Pact, the Russians thought the whole thing was on the verge of crumbling.
"And so the Brezhnev doctrine developed - if you were in the Warsaw Pact, you could not leave."
Influence of Suez
As for the influence of Suez, he said: "Moscow would have intervened anyway but it was a gift to them. It diverted attention, especially in the Third World. The Russians would have been in much worse trouble without Suez."
A recent book called Failed Illusions by Charles Gati, who escaped Hungary as a refugee and is now at the Nitze School of Advanced International Studies, says that Suez also played no role in the US refusal to intervene.
"The truth is that at a critical juncture in the Cold War, when Hungarians rose against their Soviet oppressors, the United States abandoned them," he wrote in the Washington Post.
"Washington's hands-off stance had nothing to do with the concurrent Suez crisis either. With Suez or without it, the United States had no means available to aid, let alone 'liberate', Hungary."