THE U.S. MISSION in Iraq hasn't had many good days in recent months, but yesterday was one of them. In a ceremony abruptly advanced by two days, American administrator L. Paul Bremer returned sovereignty to an Iraqi government headed by interim Prime Minister Ayad Allawi, ending in principle if not entirely in practice the 15-month U.S.-led occupation. Meanwhile, in Istanbul, NATO endorsed the new government and agreed to provide training and other assistance to Iraqi security forces. Both achievements were shadowed by the enduring problems of Iraq, above all in security, but they also widened a window of opportunity for the new Iraqi administration.

Officials were frank in acknowledging that the speeded-up transition was aimed at preempting any plans of Iraqi insurgents and foreign terrorists to disrupt the scheduled event tomorrow with another wave of attacks. Those assaults or others are surely coming, and President Bush acknowledged that Mr. Allawi may begin with "tough security measures" to combat the opposition. But polls show that Iraqis are open to supporting the new administration: Mr. Allawi would be wise to act energetically and aggressively in the coming days to rally the country behind him. Although U.S. and coalition forces will continue to carry most of the security burden, the interim government has a chance to isolate the militants politically. The more Mr. Allawi can demonstrate that his ministers, and not U.S. commanders or diplomats, are running the country, the greater his chances will be. The Bush administration can help by swearing off the sort of micromanagement practiced by Mr. Bremer, who, even in his final days, attempted to regulate matters -- from traffic regulations to electoral rules -- that would be better left to Iraqis.

NATO's offer of help, like the transfer of power, begins as a political statement with uncertain practical substance. It may be that many more European trainers will be dispatched to assist in the training of Iraqi security forces, but the details have yet to be worked out, and France and Germany, the two biggest potential contributors, remain reluctant. At a minimum, however, the alliance's decision is another step toward healing its rift over Iraq. It also offers potentially decisive political cover to those NATO governments that already have trainers or troops in Iraq and face the question of whether to leave them in place. Neither NATO nor the new government can offer a substantial substitute for the U.S. presence in Iraq anytime soon. Yet the Bush administration is a couple of small steps closer to its goal of stabilizing the country under a representative government with multilateral support.