President Barack Obama has picked two potentially controversial candidates for his second-term national security team.
Mr Brennan, a 25-year CIA veteran, withdrew from consideration for the spy agency's top job in 2008 amid questions about his connection to criticised interrogation techniques during the George W. Bush administration.
Along with secretary of state nominee John Kerry, Mr Hagel and Mr Brennan would play key roles implementing and shaping Mr Obama's national security priorities. All three must be confirmed by the Senate.
In nominating Mr Hagel, Mr Obama signalled he is willing to take on a tough confirmation fight. The 66-year-old moderate Republican has criticised discussion of a military strike by either the US or Israel against Iran. He also irritated some Israel supporters with his reference to the "Jewish lobby" in the United States. And he has backed efforts to bring Iran to the table for future peace talks in Afghanistan.
The second-ranking Senate Republican, John Cornyn, said making him defence secretary would be "the worst possible message we could send to our friend Israel and the rest of our allies in the Middle East." White House officials say Mr Hagel's positions on Israel and Iran have been misrepresented.
Both Mr Hagel and Mr Brennan have close relationships with Mr Obama, who values loyalty in his inner circle. Mr Brennan, as the president's top counter-terrorism adviser, was deeply involved in the planning of the 2011 raid that killed Osama bin Laden. And he has led administration efforts to quell the growth of terror organizations in Yemen and elsewhere in the Middle East and North Africa.
Mr Brennan, 57, served as CIA station chief in Saudi Arabia and in a variety of posts, including deputy executive director, during the Bush administration. His tenure at the agency during Mr Bush's presidency drew criticism from liberals when Mr Obama considered naming him CIA director after the 2008 election. Mr Brennan denied being involved in the Bush administration's criticised interrogation techniques but still withdrew his name from consideration. If confirmed, Mr Brennan will succeed David Petraeus, who resigned in November after admitting to an affair with his biographer.
Mr Hagel would replace retiring Pentagon chief Leon Panetta at a time when the Defence Department is facing potentially deep budget cuts. He would also be tasked with overseeing the military drawdown in Afghanistan, where the war is scheduled to end in two years.