WASHINGTON -- In promoting Army Gen. David H. Petraeus to commander of U.S. forces in the Middle East, President Bush is doing more than rewarding a job well done in Iraq. The president also is taking a step toward perpetuating his policy of high troop levels in Iraq and is putting his most trusted general in charge of renewing the military's focus on Iran.

Petraeus has been the prime advocate of Bush's policy of a large troop presence in Iraq. By naming Petraeus to a job that lasts into the next administration, Bush ensures that the new president will confront the military's strongest voice for maintaining a big force in Iraq.

And Petraeus has emerged as a leading critic of Iran's interference in Iraq, making his appointment a signal of heightened U.S. attention to Tehran. His expertise with Iran's military and political leadership will allow him to take a more hands-on approach to dealing with the government.

Defense Secretary Robert M. Gates said Wednesday that Bush would nominate Petraeus to take over as chief of U.S. Central Command, which also oversees Afghanistan and Pakistan.

The job was left vacant in March when Navy Adm. William J. Fallon stepped down abruptly after appearing to criticize U.S. policy in the region, especially in Iran.

At the same time, Gates said, Bush will nominate Army Lt. Gen. Raymond T. Odierno to take over Petraeus' current job as top U.S. commander in Iraq.

Odierno had served directly under Petraeus as day-to-day Iraq commander before stepping down in February.

Like Petraeus, Odierno has urged that Pentagon leaders and policymakers approach U.S. troop reductions cautiously to avoid creating gaps in Iraq's fragile security.

Both men must await Senate confirmation. Although confirmation hearings will be confrontational, with Democrats criticizing Bush's war policy, both men are likely to be approved.

Despite the policy disagreements, Petraeus and Odierno command wide respect because of their success in reducing the level of violence in Iraq.

During his time as Iraq commander, Petraeus has grown steadily more critical of Iran's interference in Iraq's politics and of its role in contributing to violence.

When he took command of U.S. forces in Iraq in 2007, Petraeus was surprised at the extent of Iranian meddling, said some who have worked with him.

During recent appearances in Washington, Petraeus highlighted "nefarious activities" by Iran's Quds Force and charged that the unit has armed Iraqi "special groups" that have killed U.S. troops.

"We should all watch Iranian actions closely in the weeks and months ahead, as they will show the kind of relationship Iran wishes to have with its neighbor," Petraeus told Congress.

But Petraeus also has displayed a keen understanding of the current Iranian government, and many said he would approach Tehran with reserve.

"You will find a very pragmatic general," said Frederick W. Kagan, a scholar at the American Enterprise Institute who has advised the Bush administration on its war strategy. "The Iranians won't be happy because they are not going to be able to feed him nonsense. But he won't be handing anyone in Washington memos saying, 'It's time to go to war.' "

In his new post, Petraeus will have a chance to solve a problem that, before now, he and others could only complain about.

"The question is not if Iran is unhelpful in Iraq," said P.J. Crowley, a retired Air Force colonel and a fellow at the Center for American Progress. "The question is what to do about it."

Once Bush leaves office, his successor is free to change his policies. On Iraq, the president's most important influence will be through the military officers he installs in command.

Although top officers will not set policy themselves, they will be responsible for assessing the war effort and offering the new president their best advice.