On Dec. 14, 1825, following the death of Alexander I -- who had seen off Napoleon -- his brother, the grand duke, who had just taken the oath as Czar Nicholas I, was confronted by mutinous troops and rebels in Senate Square before the Winter Palace.
For hours, the czar stood at the end of the square as the crowd shouted for a constitution or for Nicholas' brother Constantine to take the throne. Shots were exchanged.
As darkness fell, a czarist general rode up to Nicholas and said, "Sire, clean the square with gunfire -- or abdicate."
The cannons belched -- and Nicholas reigned for 30 years.
Most autocratic regimes face such moments.
After Joseph Stalin's death in 1953, East German workers rebelled, and were crushed. Rather than let the Hungarian Revolution triumph, in November 1956 Nikita Khrushchev ordered in the tanks. In August 1968, Leonid Brezhnev sent in tanks again to crush Prague Spring. In 1981, Moscow ordered Gen. Jaruzelski to smash Solidarity. Those communists did not shrink from massacre to keep what they worshipped: power.
In June 1989, Beijing, rather than let hundreds of thousands of dissidents occupy Tiananmen Square, waited for nightfall and sent in tanks and rural troops, avoiding the fate of the communist regimes of Eastern Europe.
Authoritarian rulers who recoil at bloodshed to preserve their power have not fared well.
Louis XVI let the mob lead him away from Versailles, which he never saw again. When artillery captain Bonaparte asked one of the late king's ministers why Louis had not used his cannons, the minister is said to have replied, "The king of France does not use artillery on his own people."
To which Napoleon is said to have replied, "What an idiot."
The Shah refused to use his army on the rebels and lost his throne. Mikhail Gorbachev refused to use the army to save Moscow's allies in Eastern Europe and lost the Soviet Empire.
Though Gorbachev is hailed in the West for not being a Khrushchev, no true authoritarian would regard him as a great statesman.
Tehran appears to be facing its Tiananmen moment.
Hundreds of thousands are still demonstrating against Friday's election and the regime that validated it. They are now being joined by crowds in cities where Baluchi, Arabs, Kurds and Azeris outnumber Persians, thus imperiling the unity of this diverse nation.
It is hard to believe that this theocratic regime, backed by the Revolutionary Guard and clerics, will not do whatever is necessary to preserve its power and national unity.
This is another reason President Obama is right not to declare that the United States is on the side of the demonstrators in Tehran or the other cities -- and against the regime.
Should this end in bloodshed, Obama would be blamed for having instigated it, and then abandoned the demonstrators, as Ike's U.S. Information Agency was blamed for having urged the Hungarians to rise and then left them to their fate.
When Vice President Nixon went to the bridge at Andau to welcome the Hungarian patriots fleeing the bloodbath, many cursed America for having misled them into believing we would be at their side.
If Obama cannot assist the demonstrators, why declare we are with them? That would call into question the nationalist credentials of the protesters by tying them to a power not universally loved in Iran. It would play into the hand of the regime by confirming charges that the crowds are "rent-a-mobs" like the ones Kermit Roosevelt and the CIA used to dump over the regime of Muhammad Mossadegh in 1953.
Moreover, the alternative to the Ayatollah and Mahmoud Ahmadinejad is not a republic that will renounce Islamism and Iran's nuclear program. It is ex-Presidents Khatami and Rafsanjani, and ex-Prime Minister Mir-Hossein Mousavi, all of whom trace their roots to the Revolution of 1979 and none of whom bears any great love of Uncle Sam.
It is Ayatollah Khomeini's boys versus Ayatollah Khamenei's boys. As Obama observed, in policy terms, there is no great difference.
For six days, the world has watched riveted as hundreds of thousands of Iranians peacefully protested what they believe was a stolen election, challenging the ayatollah who validated it just hours after the polls closed. For six days, the regime, born of the Islamic Revolution of 1979, has been leaking legitimacy and credibility in the eyes of its own people, the Islamic world, the whole world.
Why interfere? Why turn a widening confrontation between the Ayatollah Khamenei and the people into a spat between the president of the United States and the president of Iran?
It is impossible to believe a denunciation of the regime by Obama will cause it to stay its hand if it believes its power is imperiled. But it is certain that if Obama denounces Tehran, those demonstrators will be portrayed as dupes and agents of America before and after they meet their fate.
If standing up and denouncing the Ayatollah and Ahmadinejad from 7,000 miles away is moral heroism, it is moral heroism at other people's expense.