PRAGUE -- Last weekend's announcement that Iraqi lawmakers have finally formed a unity government is welcome news, both for Iraq and for President George W. Bush and Prime Minister Tony Blair. The American and British governments, increasingly unpopular at home, desperately needed some tangible evidence of progress to assuage their domestic critics and to begin to speak openly of an exit strategy. But Iraq's greatest challenges lie ahead. If Bush and Blair declare victory before the real battles have begun, they will undermine the very process to which both have committed so much at such great cost.

Bush waited weeks for a positive development that would allow him to suggest he can reduce troop levels in Iraq from 133,000 to 100,000 by the end of 2006. Blair, still stung by his Labour Party's defeat in local elections in early May, also welcomed the good news from Iraq. During a triumphal surprise visit to Baghdad on May 22, he said he expected Iraqi forces to take responsibility for "territorial security" in much of the country by the end of the year. "It is the violence that keeps us here," he said. "It is the peace that allows us to go."

The optimism is premature. The formation of a unity government is only the first of many hurdles Iraq's new government must clear if it is to build a durable peace. Its first task will be to remove those provisions of Iraq's constitution that pit Iraq's Sunni, Shiites and Kurds against one another. Under current Iraqi law, the parliamentary committee charged with making these constitutional changes has four months to complete its task. The four-month clock began ticking on May 3, following the first meeting of Iraq's new parliament.