BAGHDAD -- The United States began a fragile new era in its turbulent history with Iraq on Wednesday, as U.S. political and military leaders marked the official end of combat operations but acknowledged that a difficult milestone, the creation of a new coalition Iraqi government, was not yet in reach.

In the marble rotunda of Al Faw Palace, one of the lavish former palaces of Saddam Hussein that serves as the U.S. military headquarters in Baghdad, Vice President Joseph R. Biden, Defense Secretary Robert M. Gates and Gen. Ray Odierno sounded the same theme in a made-for-television ceremony to inaugurate Operation New Dawn, as the post-combat phase has been named. The U.S. military, they said, was moving toward an exit after seven years of war, but the Obama administration would not abandon the country.

"We stood together in difficult times, we fought together, we laughed together and sometimes died together," Gen. Odierno, who formally ended more than four years of service in Iraq, including two as the top U.S. commander, said during the ceremony. He said the change in mission, which still leaves about 50,000 U.S. troops in the country, "in no way signals the end of our commitment to the people of Iraq."

The ceremony was attended by hundreds of U.S. and Iraqi military commanders under U.S. and Iraqi flags hung between the rotunda's black marble columns.

Despite the pageantry of the ceremony, held the day after President Barack Obama declared combat operations at an end in a prime-time address from the Oval Office, military officials have been concerned that a prolonged political stalemate could lead to a resumption of bloodshed in Iraq, which has been sharply reduced from the dark days before a 2007 increase in U.S. forces but is still not under control.

Recent statistics gathered by the U.S. military show that in the first 17 days of Ramadan, the Muslim holy month that began in August, there was a substantial increase in casualties when compared with a similar period during Ramadan in 2009.

But in an interview, Mr. Biden played down the risk of a fresh round of violence, saying the Iraqi military was holding its own, and that it had taken the insurgents of al-Qaida in Iraq several months to organize a recent string of attacks.

Mr. Gates, who has taken a markedly anti-triumphal tone during the clamor around the end of combat, said earlier Wednesday that history had still to judge whether the U.S. involvement in the war was worth the cost.

In subdued comments to reporters in Ramadi, capital of al-Anbar province and scene of some of the war's fiercest fighting, Mr. Gates said that while U.S. service members "have accomplished something really quite extraordinary here, how it all weighs in the balance over time I think remains to be seen."

The war, he added, "will always be clouded by how it began" -- on the premise that Saddam had unconventional weapons, which did not exist.

When the Obama administration drew up the plan to reduce U.S. forces to 50,000 troops by the end of August, military planners assumed that Iraq would have a newly elected and largely representative government in place.

Although Mr. Obama's goals for Iraq are less far-reaching than those of former President George W. Bush, who envisioned a democratic Iraq that would ignite Middle East political change, the current administration's goals include an Iraqi government that, as Mr. Obama said Tuesday night, is "just, representative and accountable to the Iraqi people."

Despite the official end of the combat mission, officials say, fighting will continue. U.S. special operations forces will keep up the hunt for insurgents, along with Iraqi units. The six U.S. "advise and assist" brigades staying behind to train Iraqi forces, escort U.S. civilian advisers and protect U.N. officials have all of the weapons and military capabilities of combat units.

As the U.S. combat mission officially ended, Iraqi politicians, security officers and civil servants spoke of a daunting series of challenges they face between now and the end of 2011, when the last of the remaining American troops assisting Iraqi forces are to have departed.

At the top of the list: how to combat steadily rising violence and how to cope with the lack of a new government, six months after inconclusive national elections. Rather than move forward, parliament has met just once, and Iraq's caretaker government has stalled on projects aimed at improving people's lives.

"There are no decisions. We are just hanging now, and we have stopped everything. We are waiting for the government to make decisions," said Ghazi Abdul Aziz Essa, director-general of Baghdad's main power plant. "The delay affects the system very badly. It's not good for us."

After a government is formed, many emphasize, a mountain of problems remain. Among them: reconciliation of Iraq's ethnic and religious groups; the splitting of oil revenues, and the disputed ownership of lands now controlled by Arabs and Kurds; and an equitable revision of the nation's constitution.

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