The new security agreement with Iraq heralds an overdue end to President Bush’s ill-advised war. But while it calls for American combat forces to be out of the cities by June and all forces to withdraw from the country by the end of 2011, there is disquieting talk in Washington of having tens of thousands of troops stay longer and slyly redefining their missions.

The year ahead presents tough challenges in Iraq. The improved security environment masks pitfalls - more political than military - that could again increase tensions. We saw an example recently in the arrests that appeared to be a major crackdown inside Iraq’s internal security apparatus. Although details were not entirely clear, critics accused the prime minister, Nuri Kamal al-Maliki, of trying to intimidate political rivals before the provincial elections this month.

President Bush’s goal of a model democracy in Iraq - never realistic - remains elusive. A new report by the United States Institute of Peace found that“as the threat of state collapse has receded, the risk of an increasingly repressive and authoritarian Iraqi regime has come to the fore.”Iraq’s governing parties still resist sharing power. The administration’s unconditional support for Mr.Maliki is fueling his nationalistic rhetoric and divisive policies.

Mr.Bush and President-elect Barack Obama must do whatever they can to strengthen institutions that can stabilize Iraq, starting with ensuring that provincial elections, set for Jan. 31, and national elections, expected by the end of 2009, are as free and fair as possible. To exclude Sunnis or other minorities from this process risks a return to civil war. Iraq’s government must provide jobs for more Sons of Iraq (insurgents whose willingness to help the Americans fight Al Qaeda has been essential to declining violence) and build an army loyal to the state, not political parties.

All that takes time, but Iraqi leaders do not seem sufficiently committed to the task. American officials already are jockeying with the security agreement deadlines to compensate for Baghdad’s failings. Even Mr.Obama, who campaigned on withdrawing all combat troops within 16 months (roughly May 2010), has signaled flexibility by saying he will listen to his generals.

What are the generals saying- According to The Times’s Elisabeth Bumiller and Thom Shanker, the commanders’ new military plan would leave troops in place beyond Mr.Obama’s timetable. And there is talk about redefining solders as“trainers”or“advisers”so that they do not have to leave the cities in June. Defense Secretary Robert Gates has said that even after 2011,“several tens of thousands”of troops will remain.

If conditions deteriorate, Washington may have to slow the withdrawal pace. But, for now, we urge Mr.Obama to stick to his campaign pledge to pull combat troops out in 16 months. Some residual forces may be needed after 2011 for training and counterterrorism missions, but the number should be as small as possible. That would keep the pressure on Iraqis to assume greater responsibility for their country. The sooner Iraqis do that, the sooner all American troops can come home, an overtaxed United States military can be repaired and additional forces can be reassigned to Afghanistan, where the threat from extremists is greatest.