Despite growing opposition in Congress from both Democrats and Republicans, President Bush is calling for more troops to combat insurgents and more dollars to employ Iraqis in that violence-torn land. But Mr. Bush faces huge obstacles to success. In Congress, critics are scrambling to see what limits can be put on the fresh resources and the time allowed for them to work. As for the public, opinion polls show that a large majority of Americans doubt the tide can be turned or that American security requires it.

There are three possible outcomes. Against all odds, the escalation could tame the insurgency enough to allow political and economic developments to stabilize Iraq. Or the surge could prove to be the last, unsuccessful effort before withdrawal of most American troops, thus consigning the fate of Iraq to the Iraqis and their nervous neighbors. Or the Bush administration could struggle on inconclusively and leave the mess to the winner of the 2008 American presidential election.

For several reasons, a pessimistic outcome is likely. The Bush plan cannot work as long as the Maliki government of Iraq refuses to suppress Shiite militias, not just Sunni ones, or make the political and economic compromises sought by moderate Sunnis. Prime Minister al-Maliki has pledged to cooperate with the Bush plan, but his past behavior strongly suggests he is unable or unwilling to provide more than minimal help. He has not acted as a unity leader but rather as a Shiite advocate, with questionable ties to Iran, and has given scant cause for Sunnis to trust him.

A second reason for skepticism is that the additional troops sought by Mr. Bush are nowhere near enough to pacify Baghdad in the opinion of counterinsurgency experts. And lastly, angry and disillusioned Iraqi factions are swelling the ranks of the insurgency. For their part, the insurgents have increased the effectiveness of their tactics and demonstrated an ability to wait out counter attacks, only to reappear when the coast is clear. Mr. Bush’s new plan envisions a significantly diminished role for American troops by late fall. Based on past performance, it is doubtful that Iraq’s army will be capable by then of providing adequate security.

Unless conditions in Iraq noticeably improve, Mr. Bush will lose his remaining leverage and will finish out his presidential term isolated and marginalized. As the 2008 elections draw near, division will grow sharper within Republican ranks between supporters and doubters of escalation. And Democratic presidential candidates will feel heat from the party’s base to embrace a quick disengagement.

Within Iraq, the Bush initiative may curtail some sectarian violence but only for a while. The most realistic outlook is for civil strife between Shiites and Sunnis to rage for a number of years until there is a clear winner, a compromise brought on by exhaustion or a break up of the country. Among Iraq’s neighbors there is growing alarm over the potential for region-wide disruption. The Sunni states of Egypt, Saudi Arabia, Jordan, and the Gulf States worry most about Iran’s drive to develop nuclear weapons capabilities, its funding of radical groups and its ambitions to dominate the region. In their eyes, Iran benefited the most from the war in Iraq and America suffered the greatest loss in power and influence. With Iraq in long-term turmoil, America must work to contain the chaotic forces threatening stability throughout this oil-rich region. An essential ingredient will be an invigorated diplomacy, starting with renewed mediation of the Israeli-Palestinian conflict. Security assistance, such as missile defenses for the Gulf states, is also a priority.

The most troubling problem is Iran’s unabated push to arm itself with the capability of creating nuclear weapons at the same time that it is developing long-range missiles. Given that Iran is a radical, Shiite theocracy intent on supporting like-minded terrorist organizations, alarm bells are ringing in the neighborhood. America and Israel are insisting, so far without success, that Iran must be stopped from achieving the wherewithal to make nuclear weapons. And for their part, Egypt and Saudi Arabia are contemplating nuclear programs of their own in response to the Iranian threat. Clearly, the debacle in Iraq has undermined American led efforts to contain Iran’s nuclear ambitions and to prevent destabilization of the region. In the year ahead, no more important crisis faces America.

Mr. Hoge is the editor of Foreign Affairs at the Council on Foreign and Relations.