The new US Defence Secretary, Leon Panetta, has been sworn in.
In a letter to the Pentagon to mark the occasion, Mr Panetta pledged to maintain the "excellence" of the US military while promising "tough budget choices".
"I do not believe in the false choice between fiscal discipline and a strong national defence," he wrote.
He also reminded Pentagon staff and military personnel that "our nation is at war", and promised "we will persist in our efforts to disrupt, dismantle, and ultimately defeat al-Qaeda".
Mr Panetta - CIA director since 2009 - replaces Robert Gates, who was appointed by President George W Bush in 2006 and retained by President Barack Obama when he came to office.
The BBC reports that among his many challenges, the new defence secretary may have to manage cuts to the Pentagon budget, as the US government is currently running a US$1.5 trillion budget deficit.
Mr Panetta will also be tasked with implementing the end to the ban on gays and lesbians serving openly in the US military.
