By dumping Hosni Mubarak, America's closest and most important Arab ally, Barack Obama has signalled to Muslims throughout the Middle East that repressive rulers can no longer rely on Washington and are vulnerable to ouster by uprising.
That may be realpolitik from a pragmatic president, but the Arab street may see it as America impotent; unable or unwilling to rule from afar by propping up loyal, albeit, thuggish regimes.
Tunisia was small, oil-less, hardly strategic but - in retrospect - the first domino. Egypt, with 80-million people, keeper of the Suez Canal, partner in cold peace with Israel, and possessor of the Arab world's most powerful military, is the Arab world's strategic powerhouse and, maybe, the tipping point.
In opulent presidential palaces across the region, repressive rulers will have watched the past three history-making weeks in Tahrir Square with rising fear. Yesterday, the stark reality that Washington will not - perhaps cannot - prop up even the most loyal of potentates was hammered home.
Mr. Obama may have put himself on the right side of history, rather as Ronald Reagan did a generation ago with his ringing demand that Soviet leader Mikhail "Gorbachev, tear down this wall!"
By comparison, Mr. Obama's repeated calls for an "orderly transition" didn't quite match the rhetorical flourish. But championing freedom and democracy is risky. It can produce chaos, collapse or civil war. Every Velvet Revolution such as Prague's is matched by the violent, bloody ethnic-cleansing of a Bosnia. The Middle East's Christian and Kurdish minorities, again, face an uncertain future.
Not since the shah of Iran was abandoned by Jimmy Carter 40 years ago has a key U.S. ally been so quickly and completely dumped. The Iranian revolution of 1979 was just as popular, broad-based and full of promise as the brave, surging, pro-democracy uprising that has swept out Mr. Mubarak.
But the democratic hopes in Tehran three decades ago were soon crushed. Iran's Islamic rulers today are more repressive than the shah, avowedly hostile to the West and, with their drive to create an arsenal of nuclear missiles capable of reaching Europe, far more dangerous.
Sometimes belatedly switching allegiances mid-revolution fails to impress the revolutionaries.
For the Saudi Royal Family and Jordan's King - whose dynasties can claim to closely match Egypt's decades-long record of hand-in-glove co-operation with a parade of American presidents - the message from the Oval Office must be ominous. Both those regimes, like Mr. Mubarak's, have been Washington's faithful allies in the so-called war on terrorism. Both have crushed Islamic extremists and imprisoned those the United States fingered as enemies.
For other Arab rulers, from Libya's unpredictable Colonel Moammar Gadhafi and Yemen's already-shaky President Ali Abdullah Saleh, surviving may mean finding an exit without exile or death. Even rulers such as Syria's Bashar Assad, already hostile to Washington, must fear the contagion of popular uprisings.
There are rosy hopes that Egypt may transform itself from a repressive dictatorship, racked with poverty and reliant on a now-moribund tourism industry, into a peaceful, prosperous democracy that remains a reliable ally and a valuable partner for a wider Middle East.
But America's interests in the Middle East - oil, Israel and access to military bases - haven't changed.
If Arab world dictators - presidents, kings and emirs - topple like dominos, swept away in a tide of long-repressed anger and suffering, it may take years, even decades, for a new equilibrium to emerge.
Free and fair elections in the Arab world have been few and far between. But Hamas in Gaza and Hezbollah in Lebanon, both deemed terrorist groups by Washington and Ottawa, can justly claim to represent the political will of peoples long oppressed.
Attempting to influence, let alone manage, the cataclysm unleashed by the uprisings in Tunisia and Egypt may be the defining foreign policy challenge - created by and now facing Barack Obama.
The worst-case scenarios include the West's still-fragile economic recovery wrecked by soaring oil prices or the Gulf's vast oil reserves held hostage by Islamic regimes avowedly hostile to the United States and the rest of the West.