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.