WASHINGTON — Admiral Michael Mullen, the next chairman of the Joint Chiefs of Staff, is troubled by the Iraq war. He thinks it has become such a consuming focus of American attention that it may be overstretching the military and distracting the nation from other threats.

When the Navy officer steps into his new office in Room 2E676 at the Pentagon today, replacing General Peter Pace, a Marine, as the senior military adviser to the president and the defense secretary, Admiral Mullen already will be on record expressing his war worries with an unusual degree of candor.

“I understand the frustration over the war. I share it,” he told his Senate confirmation hearing July 31. It weighs heavily on the minds of people in America, he said, and “it weighs heavily on mine.”

As evidence of his focus on Iraq, Admiral Mullen has told Congress he intends to travel to Baghdad immediately after he takes over so he can see firsthand how the war effort is going.

Admiral Mullen, 60, was Defense Secretary Gates’s choice to replace General Pace, who had been vice chairman of the Joint Chiefs when the Iraq invasion was launched in 2003.

General Pace has been criticized by some for not speaking up more forcefully on the conduct of the war after he became chairman in October 2005. In June, Mr. Gates announced that General Pace would retire rather than serve a second term as chairman — not because of his performance in the job but because of political heat over the war.

Admiral Gregory Johnson, who retired from the Navy in December 2004 and has known Admiral Mullen for 20 years, said he believes Admiral Mullen will find ways to ensure that his views on the war are heard clearly.

“He is a sophisticated Washington player,” Admiral Johnson said in a telephone interview. “He knows how to operate in that environment, so I think he will be greatly advantaged” in the war councils.

Coming in as Mr. Gates’s choice to provide military advice gives Admiral Mullen “an incredibly strong hand,” General Johnson said. “He will play it adroitly and in a very sophisticated manner,” to the advantage of the military. Admiral Mullen arrives at a critical point in the war.

After building up American forces in the first half of this year, despite some misgivings by the Joint Chiefs, Mr. Bush now has committed to ending the increase by July.

Yet it is unclear whether Mr. Bush is any closer to the buildup’s ultimate goal of getting the Iraqi government to move toward a peaceful reconciliation.

If the picture is still murky in July, will Mr. Bush proceed with further troop cuts? That is the kind of decision in which Admiral Mullen’s view will carry weight. As the chief of naval operations for the past two years, Admiral Mullen had a lesser role in the conduct of the war, given that most of the fighting is done by soldiers and Marines. Even so, he has let it be known that he is troubled by the broader effects of an escalating military commitment in Iraq.

“I worry about the toll this pace of operations is taking on [the troops], our equipment, and on our ability to respond to other crises and contingencies,” Admiral Mullen told the Senate Armed Services Committee.

He has made it clear that he agrees with a central tenet of the current American strategy in Iraq — that establishing security is critical to giving the Iraqi government the “breathing space” it needs to find a power-sharing formula. But he also sees limits to how long the military can wait. Political reconciliation and economic growth are equally important to stabilizing Iraq, he said. “Barring that, no amount of troops and no amount of time will make much of a difference,” he told the committee.

Admiral Mullen also has emphasized his concern that strains from the long wars in Iraq and Afghanistan may detract from the country’s ability to handle threats elsewhere.

“To the degree that we narrow our focus solely on those two pieces of the overall global puzzle, we lose sight of other state and non-state threats in the region and around the world,” he said in remarks to a conference on national security last week. He mentioned North Korea as a missile threat.

Admiral Mullen’s concerns about the impact of the prolonged war on troops and their families is in line with Mr. Gates’s thinking. Together, they might be expected to push for a quicker drawdown of American troops in the second half of next year than General David Petraeus, the top commander in Iraq, would like.