“This is the neocons’ worst nightmare because you’ve got a combat soldier, successful businessman and senator who actually thinks there may be other ways to resolve some questions other than force,” said Richard L. Armitage, who broke with the more hawkish members of the Bush team during the Iraq war when he was a deputy to Secretary of State Colin L. Powell…

In fact, the neoconservatives have done anything but disappear. In the years since the war’s messy end, the most hawkish promoters have maintained enormous sway within the Republican Party, holding leading advisory posts in both the McCain and Romney presidential campaigns as their counterparts in the “realist” wing of the party, epitomized by Mr. Powell, gravitated toward Barack Obama…

“Here was a Republican with national security credentials saying that the Republican president was being irresponsible on national security — that’s potent,” said Kenneth L. Adelman, a member of the Defense Policy Review Board at the time and a frequent sparring partner with Mr. Hagel on television. “It drove me up the wall not so much that he was Republican, because I didn’t care that much from a political point of view — I thought the substance of his arguments were just wrong and unfounded.”

Mr. Hagel’s earliest concerns arose before the Congressional vote authorizing the use of force. “You can take the country into a war pretty fast,” he said in an interview with The New York Times in 2002, “but you can’t get us out as quickly, and the public needs to know what the risks are.” In the interview, he took a swipe at Mr. Perle, then one of the most visible promoters of the war, saying, “Maybe Mr. Perle would like to be in the first wave of those who go into Baghdad.”