The new top military adviser to US President George W Bush has said he wants to prepare the military for challenges beyond Iraq and Afghanistan.
Adm Mullen has expressed doubts over the war in Iraq
At his swearing-in ceremony as chairman of the Joint Chiefs of Staff, Admiral Michael Mullen said he wants to revitalise the armed forces.
Meanwhile, the FBI is to investigate US security firm Blackwater over the deaths of 11 Iraqis last month.
US politicians are also to question the company's chairman about the incident.
Iraq war doubts
At a ceremony at Ft Myer, Virginia, presided over by President Bush, Admiral Mullen said: "The fighting in Iraq and Afghanistan will one day end.
"We must be ready for who and what comes after."
Adm Mullen takes over from Gen Peter Pace.
BLACKWATER USA FACTS
Founded in 1997 by a former US Navy Seal
Headquarters in North Carolina
One of at least 28 private security companies in Iraq
Employs 744 US citizens, 231 third-country nationals, and 12 Iraqis to protect US state department in Iraq
Provided protection for former CPA head Paul Bremer
Four employees killed by mob in Falluja in March 2004
He said he wants to plan to support US interests in the Middle East and "to reset, reconstitute and revitalise our armed forces, especially our ground forces".
Some military officials have said the focus on counter-insurgency warfare in Iraq and Afghanistan has left the military unprepared for other types of operations.
Admiral Mullen has openly expressed doubts over the war in Iraq.
His swearing-in came as the FBI confirmed it is to send a team of investigators to Iraq to look into Blackwater's role in the shooting deaths of 11 Iraqi civilians in September.
The Iraqi government says Blackwater agents opened fire without provocation. The company says insurgents attacked first.
In Washington on Tuesday, a Congressional committee is to question the firm's chairman, Erik Prince, as part of a hearing into the role of private security firms in Iraq.
A report by the US Congress reveals the company fired 122 people in three years for problems ranging from violent behaviour to drug use.
The report says Blackwater employees were involved 195 shooting incidents in Iraq from the beginning of 2005 until 12 September this year, causing 16 Iraqi casualties.
The report does not say how many of these were deaths.
There is concern about how private security firms, especially Blackwater, operate in Iraq.