Anyways, to learn more, I found Peter Beinart's article in the Daily Beast informative, this passage in particular:
What makes Hagel so important, and so threatening to the Republican foreign-policy elite, is that he is one of the few prominent Republican-aligned politicians and commentators (George Will and Francis Fukuyama are others, but such voices are rare) who was intellectually changed by Iraq. And Hagel was changed, in large measure, because he bore within him intellectual (and physical) scar tissue from Vietnam. As my former colleague John Judis captured brilliantly in a 2007 New Republic profile, the Iraq War sparked something visceral in Hagel, as the former Vietnam rifleman realized that, once again, detached and self-interested elites were sending working-class kids like himself to die in a war they couldn’t honestly defend. It is certainly true that some politicians who served in Vietnam—for instance, John McCain—did not react to Iraq that way. But it is also true that the fact that so few American politicians and pundits lived the kind of wartime hell Hagel endured made it easier for them to pass through the Iraq years unscathed.If true, this is an encouraging development. Here's hoping this country can take the first step towards reigning in the military/industrial complex, without which any real debt-management effort will be handicapped.
But Iraq and Afghanistan have convinced Hagel that boosting American military spending, and extending America’s global military footprint, can weaken national security if they drive America deeper into debt. Like his hero, Eisenhower, who slashed defense spending because, according to his Treasury secretary, he “feared deficits almost more than he feared the communists,” Hagel believes the defense budget must “be pared down,” because he refuses to divorce the conversation about military spending from the conversation about fiscal solvency.