Washington ? After last week’s murder of four American civilian contractors in Fallujah, U.S. leadership in Baghdad promised that the response against that city would be “precise” and “overwhelming.” But precisely who is to be overwhelmed, and what will be the metric of success at overwhelming? How many troops will it take to find those involved in the killing of the contractors? And on the basis of what intelligence?

As this is written, headlines speak of 1,200 Marines “encircling” Fallujah, which is as populous as Newark, N.J. It is a sign of things falling apart that common language seems unable to get a purchase on Iraq’s new reality — a civil war defined by the uprising of many Shiites against the U.S. occupation.

Nothing in America’s national experience is comparable to today’s dependence on the good will of a reclusive 73-year-old Shiite ayatollah, Ali Sistani. That dependence would be ominous enough if he were the uncontested voice of Iraq’s Shiite majority. But now his 30-year-old rival, Moqtada Sadr, has summoned his followers to “terrorize your enemy” — America.

By proclaiming himself allied with two terrorist organizations — “I am the beating arm for Hezbollah and Hamas here in Iraq” — he compelled U.S. commanders to seek his arrest, which would mean martyrdom in the eyes of his followers. In the war against the militias, every door American troops crash through, every civilian bystander — there will be many — shot, will make matters worse, for a while. Nevertheless, the first task of the occupation remains the first task of government — to establish a monopoly on violence.

When Sadr’s forces took to the streets, with assault rifles and rocket-propelled grenade launchers, many of the freshly minted Iraqi security forces took flight. It is too late for debate about being in Baghdad. And the (relatively) pretty phase of empire — the swift dispatch of an enemy army — is over. Regime change, occupation, nation-building — in a word, empire — is a bloody business. Now Americans must steel themselves for administering the violence necessary to disarm or defeat Iraq’s urban militias, which replicate the problem of modern terrorism — violence that has slipped the leash of states.

For the near term, U.S. policy must flow from Napoleon’s axiom: “If you start to take Vienna — take Vienna.” We started to take Iraq 13 months ago. That mission is far from accomplished.

A U.S. official in Baghdad accurately insists that the violent insurgency involves “a minuscule percentage” of the 25 million Iraqis. However, history usually is made not by majorities but by intense minorities. Remember 1917, and this from Richard Pipes’ “The Russian Revolution”: “The Bolshevik triumph in October was accomplished nine-tenths psychologically: the forces involved were negligible, a few thousand men at most in a nation of one hundred and fifty million.” There may have been fewer Bolsheviks than there are members of Sadr’s militia, which is one of many. The cancellation last weekend of a Baghdad trade fair was symbolic of the ability of a minuscule minority to sow chaos sufficient to prevent a majority from attending to mundane matters.

Not much else having gone as planned since the fall of Baghdad, a delay in the transfer of sovereignty, scheduled for June 30, should not be unthinkable. A delay would trigger violence. But, then, the transfer on schedule probably would be preceded by an offensive by the insurgents. The transfer is to be from the Coalition Provisional Authority, whose authority does not extend throughout the country. A U.S. official in Baghdad says Sadr will be arrested if he appears “any place that we control.”

The transfer is to be to an institutional apparatus that is still unformed. This is approaching at a moment when U.S. forces in Iraq, never adequate for postwar responsibilities, are fewer than they were.

U.S. forces in Iraq are insufficient for that mission; unless the civil war is quickly contained, no practicable U.S. deployment will suffice. U.S. forces worldwide cannot continue to cope with Iraq as it is, plus their other duties — peacekeeping, deterrence, training — without stresses that will manifest themselves in severe retention problems in the reserves and regular forces.

Since 9-11, Americans have been told that they are at war. They have not been told what sacrifices, material and emotional, they must make to sustain multiple regime changes and nation-building projects. Telling such truths is part of the job description of a war president.

George Will is a columnist for Washington Post Writers Group.