Army Gen. David Petraeus, who presided over Iraq's pullback from the brink of all-out civil war, relinquished his command Tuesday to Gen. Ray Odierno under a cascade of official thank-yous.

"When Gen. Petraeus took charge 19 months ago, darkness had descended on this land," Defense Secretary Robert M. Gates said in a ceremony marking the occasion in Baghdad. "Merchants of chaos were gaining strength. Death was commonplace. Around the world, questions mounted about whether a new strategy -- or any strategy, for that matter -- could make a real difference."

In an elaborate hand-off in a marble-lined rotunda of a former Saddam Hussein palace on the outskirts of the capital, Petraeus gave Odierno the responsibility for leading U.S. and coalition forces at a stage in the still-unpopular war that appears far more hopeful than when Petraeus assumed command.

Petraeus leaves behind a heavy dose of caution, reflected in his recommendation to President George W. Bush that he maintain 15 combat brigades in Iraq through the end of the year instead of pulling out one or two, as many had expected.

Still, military leaders saw reasons for optimism.

"Here we are, 18 months later, and Iraq is a vastly different place," Navy Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, said during the ceremony. "Attacks are at their lowest point in four years, 11 of 18 provinces have been turned over -- including the once-written-off Anbar province -- to Iraqi security forces, who are increasingly capable and taking more of a lead in operations."

The Iraqi government is providing for its people, the Legislature is passing laws and the courts are enforcing justice, the chairman said. "In more places and on more faces we are seeing hope; we see progress," the admiral said.

Mullen said he looks forward to working with Petraeus as the general takes over the reins of U.S. Central Command next month.

Odierno, who is beginning his third tour in Iraq, served under Petraeus as commander of Multinational Corps Iraq. Petraeus will take command of U.S. Central Command in late October.