US Marine Corps Gen Peter Pace has been sworn in as chairman of the Joint Chiefs of Staff, the country's highest ranking military officer.
Gen Myers (L) swore in his successor at a retirement ceremony
He succeeds Air Force Gen Richard Myers, who is retiring after 40 years' service in the military.
Gen Pace, previously deputy chairman, is the first marine to hold the post.
He is seen as a close ally of Defence Secretary Donald Rumsfeld, and played a key role in shaping strategy for the US-led war on terror.
The Joint Chiefs chairman is the top military adviser to the president and the defence secretary, but has no troops under his command.
Gen Pace is regarded as a low-key figure without much of a public profile. He has seen active service in Vietnam and Somalia.
Correspondents say he will now be faced with the considerable task of implementing the modernisation programme in a context of US budget deficits.
'Leadership and flexibility'
Gen Myers swore in his successor at a retirement ceremony at Fort Myer outside Washington DC.
President George W Bush and members of his administration attended the event, which included a flyover by fighter aircraft and a 19-gun salute.
Mr Bush praised the outgoing chairman, who took up his post weeks after the 11 September 2001 attacks.
"His leadership and flexibility were essential to the liberation of Iraq, and to adapting our tactics to defeat the terrorists and help Iraqis build a peaceful democracy," he said.
He said he was looking forward to working with his successor.
Gen Pace said there was plenty of work to do.
"We have an enemy whose stated, public intent is to destroy our way of life. 2.4 million Americans in uniform say: not on our watch," he said.