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.
The Turks, Saudis, Qataris and Kuwaitis who, stupidly, have been aiding ISIS in bringing down Bashar Assad and blowing a hole in the "Shia Crescent" of Tehran, Baghdad, Damascus and Hezbollah, have lately awakened to their idiocy and are cutting off aid to ISIS.
Moderate Sunnis detest ISIS for its barbarism and desecration of shrines. The Christians and Yazidis fear and loathe them. The Kurds, both the Syrian YPG and PKK, which broke open the exit route for the Yazidis from Mount Sinjar, and the peshmerga despise ISIS.
Lebanon's army, Syria's army, Hezbollah and Iran have been fighting ISIS with Russian assistance. Vladimir Putin himself warned us of the absurdity of our attacking Assad last year, arguing that we would be allying ourselves with the same terrorists who brought down the twin towers.
Was Putin not right?
Even al-Qaida and Hamas have repudiated ISIS.
We need no boots on the ground in Syria, for it is the presence of "Crusaders" on Islamic soil that is the principal recruiting tool of the jihadists.
What we need is diplomacy beyond the simple-minded, "Either you are with us or you are with the terrorists!" a diplomacy that invites old enemies into a coalition for a cause on which we all agree.
If Assad is willing to go in for the kill on ISIS, let us work out a truce and amnesty for the Free Syrian Army and call off that part of the rebellion, so Assad's army can focus on killing ISIS.
George H. W. Bush made an ally of Hafez al-Assad in Desert Storm.
Why not make an ally of his son against ISIS?
We should next tell the Saudis, Qataris and Kuwaitis that any more aid to ISIS and they are on their own. We should inform the Turks that their continued membership in NATO is contingent upon sealing their border to ISIS volunteers and their assistance in eradicating the terrorist organization.
We should convey to Iran that an end to our cold war is possible if all attacks on the West stop and we work together to exterminate the Islamic State. Why would they not take the deal?
As for Abu Bakr al-Baghdadi, the self-proclaimed successor to Muhammad, my bet is that he closes out his brief career as caliph at an unscheduled meeting with Seal Team 6.