If Mr. Bush does persist in sending more American troops to Baghdad, despite Congress’s amply justified opposition, he will have to establish clear lines of command that assure that those troops can enter the strongholds of the Shiite militias responsible for much of the violence without militia leaders’ being tipped off by allies in the Iraqi government.

And so long as any American troops remain in Iraq, Mr. Bush must put serious pressure on Mr. Maliki to support the troops’ efforts with a genuine program of national reconciliation. That must include, at a minimum, ridding the police and other security services of killers, torturers and criminals and disarming all sectarian militias.

The government must also assure that Iraqi oil revenues are fairly shared out among the entire Iraqi population. And it must move quickly to offer an amnesty to Sunni insurgents willing to put down their weapons, and narrow the legal restrictions on former Baath Party members so that Sunni professionals can once again fully participate in Iraqi national life.

These benchmarks should be accompanied by fixed timelines. And they must be accompanied with a clear message that the United States is prepared to withdraw its troops if the Iraqis continue to refuse to take responsibility for their own future. Mr. Bush and other American officials need to make clear that as much as the United States will suffer from a complete collapse in Iraq, Iraq’s leaders will suffer far worse from the loss of their American protectors.

Mr. Bush should reinforce that message by convening a conference of all of Iraq’s neighbors to discuss how they can help stabilize Iraq — and what they can do to contain the wider chaos should it come. With nearly two million Iraqis already seeking refuge, mainly in Syria and Jordan, it is far past time for American officials to begin their own planning and relief efforts.

If Mr. Bush refuses to deliver this ultimatum to Mr. Maliki, Congress will have to do so in his stead. That’s not the usual division of labor between the executive and legislative branches, but it is one that Mr. Bush has made necessary by his refusal to face realities. The potential consequences of his failed leadership are so serious that neither the new Democratic majorities in Congress, nor the public at large, can afford the luxury of merely criticizing from the sidelines.

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So far, Congress is off to an encouraging start, holding substantive oversight hearings and asking probing questions of administration officials for the first time in too many years. Similarly encouraging has been the bipartisan character of this reinvigorated oversight. The Congress should continue asking hard questions. And it must insist on real answers before acting on any new requests for money to support Mr. Bush’s plans to send more troops to Baghdad. Congress has the authority to attach conditions to that money, imposing benchmarks and timetables on Mr. Bush, who then would be forced to impose them on the Iraqi government.