Leon Panetta will be sworn in as US defense secretary on Friday at a private ceremony at the Pentagon, where a looming budget battle may be as big a challenge as the wars he will inherit.
Panetta, who is scheduled to arrive at the Pentagon at 8.30am EDT (1230 GMT) and take the oath of office 15 minutes later, has served as CIA director for the past two and a half years, at a time when the lines between US covert and military operations have blurred.
He comes to the Pentagon two months after a covert raid killed Osama bin Laden in Pakistan, and a week after President Barack Obama announced a faster-than-expected US troop drawdown in Afghanistan.
The 73-year-old Panetta has a full first day in office, including attending a meeting with the chairman of the joint chiefs of Staff and service chiefs in "The Tank," the ultra-secure Pentagon briefing room.
He is also expected to issue a message to defense department personnel and troops at a time when pressures to cut defense spending in Washington has raised concerns about a possible hollowing out of the US military, despite the wars in Afghanistan and Iraq and operations in support of the Nato mission in Libya.
"The budget will be an important item on his agenda. He will take that very seriously," said Douglas Wilson, assistant secretary of defense for public affairs.
"He knows that there are difficult decisions to be made. He has said publicly and he will say again that he intends that there will be no hollow force on his watch."
Obama has called on the defense department to come up with $400 billion in reductions over 12 years as he struggles to reduce the country's $1.4 trillion deficit and $14 trillion debt.
In an interview with Reuters this week, outgoing Defense Secretary Robert Gates said military spending was not the cause of the budget deficit. Even a "disastrous" 10 percent cut would reduce the budget shortfall only by some $50 billion - about 4 percent, he noted.
"We are not the problem," said Gates, who is retiring after four and a half years in the job.