For the Kaiser's Germany, it was the "blank cheque" to Austria after Sarajevo. For Great Britain, the 1939 war guarantee to Poland. For the Third Reich, it was the June 1941 invasion of Russia. For the Empire of the Sun, the decision to attack Pearl Harbor.
And for the Soviet Empire, it was the invasion of Afghanistan.
As for the United States, historians may one day concur with the late Gen. Bill Odom. For the lone superpower to survive that century, the decision to invade and occupy Iraq was the most disastrous blunder in its history.
George W. Bush held out the promise of a peaceful Mesopotamian democracy as a magnet for all Arab nations. What we produced is a broken land awash in blood, a country severed by tribe and faith: a Kurdish north, Shia south and a Sunni west controlled by the savages of an "Islamic State" even al-Qaida hates and fears.
In Syria, where the United States has been aiding rebels to bring down Bashar Assad, that Islamic State now controls the northern and eastern half of the country. In Libya, where we delivered the air and missile strikes to smash Col. Gadhafi's forces, Islamist fanatics have gained the upper hand in the civil war for control of that country.
In all three countries, the United States, which claimed to be battling dictatorship to bring democracy, helped to create the power vacuum these Islamists have moved to fill.
We are the enablers of the Islamic State.
How grave is the threat?
ISIS is a "direct threat to our homeland" says Rep. Peter King. "An existential threat" echoes Sen. Lindsey Graham, "I think of an American city in flames."
The Islamic State "is beyond anything we've seen," says Sec. Chuck Hagel, an "imminent threat to every interest we have."
America is "in the most dangerous position we've ever been in," says Sen. Jim Inhofe, "They're crazy out there. And they are rapidly developing a method to blow up a major U.S. city."
Undeniably, these are bloodthirsty religious fanatics who revel in beheadings and crucifixions and have exhibited battlefield bravery and skill.
But are 17,000 jihadi fighters in landlocked regions of Iraq and Syria really an imminent and mortal threat to an America with thousands of nuclear weapons and tens of thousands of missiles and bombs and the means to deliver them?
How grave is this crisis? Consider the correlation of forces.
Who are the vocal and visible friends and fighting allies of ISIS?
They are nonexistent.