(Adds Democrats, paragraph 6, Bush paragraph 9, edits)

By Andrew Gray

WASHINGTON, June 8 (Reuters) - The top U.S. military officer, Gen. Peter Pace, will retire because any attempt to renominate him would have been a "divisive ordeal" focused on Iraq, Defense Secretary Robert Gates said on Friday.

Gates picked Adm. Mike Mullen, the head of the U.S. Navy, to replace Pace as chairman of the Joint Chiefs of Staff after the Marine general's two-year term ends in September.

Gates said he had wanted to keep Pace in the post for another term but, in the latest sign of the deep divisions caused by the Iraq war, concluded his reconfirmation hearings in the U.S. Senate would have been too contentious.

"I have decided that at this moment in our history, the nation, our men and women in uniform and General Pace himself would not be well served by a divisive ordeal in selecting the next chairman of the Joint Chiefs of Staff," Gates said.

"I wish it were not necessary to make a decision like this. But I think it's a realistic appraisal of where we are."

Harry Reid, leader of the Democratic majority in the Senate, said senators would be looking closely to see if Mullen was committed to a change of course in the war.

The chairman of the Joint Chiefs is the top military adviser to the president. Pace served two terms as vice chairman before taking the top job, meaning he has held a key role during the wars in both Iraq and Afghanistan.

Gates said hearings to confirm Pace for another term would focus on the past. "There was the very real prospect the process would be quite contentious," he said.

The White House said President George W. Bush had agreed with Gates reluctantly "because he has the highest respect for Gen. Pace."

Gates himself took office as a result of divisions over Iraq. He replaced Donald Rumsfeld, a lightning rod for criticism of the war, after voters angry about Iraq put Democrats in control of Congress last November.

NO RUMSFELD

Pace is generally not a combative and outspoken figure like Rumsfeld. He showed a mild-mannered, friendly demeanor and deferred to his political bosses in public settings.

But politicians and retired generals have accused Pace and other top officers of failing to have a proper post-invasion plan for Iraq and using too few troops -- or failing to stand up to the administration on those issues.

More than 3,500 U.S. troops and many tens of thousands of Iraqis have died since U.S.-led forces invaded Iraq in 2003 to topple Saddam Hussein.

In a rare case where his comments caused controversy, Pace said earlier this year he believed homosexual acts were immoral. He later said he should have focused less on his personal views, but did not apologize.

Mullen, Pace's likely successor, became Chief of Naval Operations in July 2005. His previous assignments include a spell as commander of U.S. naval forces in Europe.

"I have become well acquainted with Admiral Mullen over the past six months and believe he has the vision, strategic insight, experience and integrity to lead America's armed forces," Gates said.

Gates also said he had recommended Marine Gen. James Cartwright, currently the head of U.S. Strategic Command, as vice chairman of the Joint Chiefs.

Cartwright would replace Navy Adm. Edmund Giambastiani, who said last week he was planning to retire later this year.

Under the process for senior military appointments, Gates makes recommendations to Bush, who then makes nominations for confirmation by the Senate.

Senate Armed Services Committee Chairman Carl Levin said he sought views from other senators on renominating Pace at Gates' request and many felt any hearing "would have been a backward-looking debate about the last four years."

He called Mullen "well-qualified".

Other senators praised Pace's military career but said they looked forward to working with Mullen and Cartwright.

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