"After a year of Vietnam's miserable heat, nearly constant danger, and violent campaigns like the Tet Offensive, Chuck Hagel came back to the United States ready to get on with things -- and with both a loyalty to the U.S. military and a belief he should do all he could to prevent his nation's being involved in another war."

His fierce opposition to the Iraq War went far toward creating the schism that now exists between him and the GOP establishment.

"The damage this war has done to our country will play out for years to come," he wrote in his 2008 book, "America: Our Next Chapter."

"While it is easy for nations to blunder into war, they never blunder into peace," he added.

Especially on foreign policy issues, Hagel has shown time and again his willingness to speak his mind -- even if it goes against the prevailing Republican thinking or, in the case of the 2009 Afghanistan surge, the position of Obama.

His independence has spawned critics, as well as cheers.

"He's a guy with really serious foreign policy chops and someone, frankly, who hasn't been afraid to depart from his party when he thought they were wrong," Murphy, the Democratic senator from Connecticut, told CNN.

Hagel himself said Monday that he won't hold back if he becomes defense secretary, telling Obama, "I will always give you my honest and most informed council."