"Chuck knows that war is not an abstraction," Obama said. "He understands that sending young Americans to fight and bleed in the dirt and mud, that's something we only do when it's absolutely necessary."
The president, who prefers to keep a tight inner circle, has close ties to both Hagel and Brennan. Obama and Hagel served together in the Senate and made several trips overseas. Brennan has become one of the president's most trusted advisers, working with him during the planning of the raid that led to the death of Sept. 11 mastermind Osama bin Laden and holding broad authority over the administration's counterterrorism operations.
If confirmed, Hagel and Brennan will join Secretary of State nominee John Kerry as Obama's key national security advisers in his second term. Kerry, a longtime Democratic senator from Massachusetts, is expected to be easily confirmed by his Capitol colleagues.
At the top of Obama's national security agenda will be winding down the U.S.-led war in Afghanistan by the end of 2014. Key decisions on troop withdrawals are looming, and Hagel is expected to favor a more rapid drawdown than some generals have suggested. Brennan, having served as Obama's top counterterrorism aide for the past four years, would bring to the CIA a deep understanding of al-Qaida in the region.
For Brennan, Monday's nomination represents a second chance at the spy agency's top job after his withdrawal from consideration in 2008.
In a letter to Obama at that time, Brennan said he was "a strong opponent of many of the policies of the Bush administration, such as the pre-emptive war in Iraq and coercive interrogation tactics, to include waterboarding."
White House officials say they don't expect Brennan to face similar trouble this time around given his four years of service in the Obama administration.
"The issue has been removed from the debate because the president and John Brennan, as his top counterterrorism adviser, brought those techniques to an end," said Ben Rhodes, Obama's deputy national security adviser.
But some lawmakers and outside groups aren't as easily convinced. Sen. John McCain, R-Ariz., issued a statement about Brennan saying he had "many questions and concerns about his nomination to be director of the Central Intelligence Agency, especially what role he played in the so-called enhanced interrogation programs while serving at the CIA during the last administration, as well as his public defense of those programs."
Brennan's nomination will also put a spotlight on the administration's controversial drone program. He was the first Obama administration official to publicly acknowledge the highly secretive targeted killing operations, defending the legality of the overseas program and crediting it with protecting American lives and preventing potential terror attacks.
The American Civil Liberties Union voiced its concerns over Brennan's nomination Monday, saying the Senate should not move forward until it is clear the nominee "will end its targeted killing program."
Brennan would replace Michael Morrel, the CIA's deputy director who has been acting director since David Petraeus resigned in November after admitting to an affair with his biographer. Hagel would replace retiring Pentagon chief Leon Panetta.Associated Press writers Donna Cassata and Tom Raum contributed to this report.