And of course, there is Mr. Kristol himself, who in the late 1990s helped form a group called the Project for a New American Century. In 1998, the organization released a letter to President Bill Clinton arguing that Saddam Hussein posed a potential nuclear threat to the United States, Israel and moderate Arab states and should be ousted.

It was signed by several future members of the Bush national security team: Donald H. Rumsfeld, who served as defense secretary; Paul D. Wolfowitz, who served under Mr. Rumsfeld; Mr. Abrams; and outsider advisers, including Richard N. Perle, a former chairman of the Defense Policy Board Advisory Committee; and Mr. Armitage. Serving as a research associate was Michael Goldfarb, who is helping to direct the Emergency Committee for Israel’s attacks against Mr. Hagel.

Around the same time in the late 1990s, Mr. Hagel was allied with Mr. Kristol and other hawks calling for the commitment of ground troops in support of the Clinton administration’s intervention in Kosovo. Mr. Kristol went so far as to suggest Mr. Hagel as a potential running mate for Mr. Bush in 2000, calling him an “impressive and attractive first-term senator.”

Their relationship broke with Mr. Hagel’s criticism of the Iraq war, and his rare status as a Congressional Republican critical of the intervention led to plentiful TV bookings and the antipathy of the war’s architects and supporters. Besides being a member of the Senate Foreign Relations Committee, Mr. Hagel had added cachet by way of two Purple Hearts from his service in Vietnam, which left shrapnel embedded in his chest and, he has said, a unique perspective on war.

“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.”

Mr. Perle had never served in the military. Along with Mr. Hagel’s comment in Newsweek that many of the war’s most steadfast proponents “don’t know anything about war,” his criticism prompted a national discussion about “chicken hawks,” a derisive term for those advocating war with no direct experience of it. And his comments drew a rebuke from The Weekly Standard that Mr. Hagel was part of an “axis of appeasement.”