US President Barack Obama has named Chuck Hagel to be his next defence secretary and counter-terrorism adviser John Brennan to lead the CIA, but the nominations may not go smoothly.
Former Nebraska Senator Hagel's fellow Republicans have accused him of being hostile to Israel and soft on Iran.
Mr Brennan is also under scrutiny over harsh interrogation techniques used at the CIA.
Both appointments must be confirmed by the Senate.
Mr Obama, who has just returned from a family holiday in Hawaii, said at a White House press conference that Mr Hagel was "the leader our troops deserve".
'Worst possible message'
Mr Obama said that Mr Hagel, 66, has been a "champion of our troops", as he praised his independence and bipartisan approach.
The president said Mr Hagel knew that American leadership was "indispensable", but added that he would treat military action as a last resort.
Mr Obama said: "Most importantly, Chuck knows that war is not an abstraction."
Mr Hagel, a decorated Vietnam War veteran, would be the first enlisted soldier to lead the Pentagon.
In his remarks, Mr Hagel said he would try to "live up to the standards" of his predecessors, as he pledged to strengthen America's alliances.
Departing Defence Secretary Leon Panetta said he strongly supported Mr Hagel's nomination.
"I believe his experience, his judgment and his deep understanding of the security issues facing this country make him the right choice to be the next secretary of defense," Mr Panetta said in a statement.
Meanwhile, Mr Brennan said he would work to ensure that the CIA "always reflects the liberties, freedoms and values that we hold so dear".
Along with Senator John Kerry, whom Mr Obama nominated last month to replace Hillary Clinton as secretary of state, Mr Hagel and Mr Brennan would help shape the president's second-term national security agenda.
But the choice of Mr Hagel could prompt a Senate confirmation battle.
Mr Hagel has stoked controversy in criticising a military strike by either the US or Israel against Iran. He has also advocated including Iran on future peace talks in Afghanistan.
Although no Republican lawmakers are threatening to block Mr Hagel's nomination, influential senators have attacked him.
Senator John McCain said he had "serious concerns" over the Nebraskan's positions on a "range of critical national security issues", which he would raise during the Senate confirmation process.
Mr Hagel made critical remarks against the Israel lobby in the US capital, in a 2008 book by former state department official Aaron David Miller.
"The Jewish lobby intimidates a lot of people up here," Mr Hagel was quoted as saying. "I'm a United States senator. I'm not an Israeli senator."
Top Republican Senator Lindsey Graham told CNN on Sunday: "This is an in-your-face nomination of the president to all of us who are supportive of Israel."
But White House officials say Mr Hagel's positions on these issues have been misrepresented, saying he voted to send billions in military assistance to Israel and has supported the imposition of multilateral sanctions on Tehran.
In an interview with his hometown newspaper on Monday, the Lincoln Journal Star, Mr Hagel said his record showed "unequivocal, total support" for Israel and that his critics had "completely distorted" his record.
Mr Hagel has also been criticised by some Democrats for saying in 1998 that a nominee for an ambassador post was "openly, aggressively gay". He has since apologised for those comments.
President Obama's decision to nominate John Brennan to lead the Central Intelligence Agency is also not without controversy.
Although put forward for the same role in 2008, Mr Brennan withdrew his name amid questions about his connection as a top CIA official to interrogation techniques used during the administration of George W Bush.
Sen McCain said in a statement on Monday he had questions for Mr Brennan, "especially what role he played in the so-called enhanced interrogation programs... as well as his public defense of those programs".
A CIA veteran, Mr Brennan is currently Mr Obama's chief counter-terrorism adviser.
The 57-year-old was heavily involved in the planning of the 2011 raid that killed Osama Bin Laden.
He would replace Gen David Petraeus, who resigned in November after admitting to an affair with his biographer.
In a memo to staff, acting CIA director Michael Morell called Mr Brennan an "outstanding choice", citing his two decades of experience at the agency.