Matthew Yglesias links to Fred Kaplan, who thinks that the US military is repeating the same mistakes as HR McMaster detailed in his survey of the Vietnam War, Dereliction of Duty. (I read DoD last winter, with posts 1,2,3,4 to show for it.) There's a number of things that we can say about this issue, but at root there's the problem that Caesar would have understood: there's an inescapable tension when the people who hold the weapons of state are not also the people calling the shots. Modern democracies pretend to have squared this circle, but we really haven't.
Matt says that even when Generals do raise their voices (citing Gen. Eric Shinseki) they're ignored, and not just by the President. He's right, and this is the broader argument that McMaster makes, but also Tim Weiner in Legacy of Ashes, his new history of the CIA. The President policy goals inevitable shape the advice he's given, not the other way around. In DoD, McMaster notes repeatedly how Generals couched their advice to Johnson and McNamara to fit the overall plan (escalation in Vietnam) while hoping, sometime in the vague future, to get the war they wanted. Weiner notes a number of occasions where the CIA provided the intelligence the White House wanted, not the intelligence that the evidence supported, and yes this goes back well before Iraq II.
Secondly, there's the problem of the advice that Generals actually provide. In DoD, McMaster makes it clear that the advice the General staff were witholding from their President was that they believed the Vietnam War could only be won with a land invasion of North Vietnam and unrestricted bombing of Hanoi and Haiphong. They weren't saying this because they were trying to scare LBJ away from his policy goal -- they sincerely wanted a land invasion of North Vietnam, and hoped Johnson would call up sufficient forces and economic measures to make that possible. This was also an insane idea, and would only have added to the body count without "winning" America anything in the process. So when Matt says that Rick Shinseki was trying to warn us away from Iraq by talking about an occupation force of hundreds of thousands, I'm not so sure. It's equally plausible that Shinseki, in the tradition of the Army, simply expected Bush to make those forces available. Shinseki hasn't talked publicly about Iraq since leaving from the Army, but I think the "scaring us sensible" explanation for Sinseki's remarks is problematic.
Finally, we don't only need to listen to Generals. The biggest problem with the run up to the war in Iraq was not that we weren't warned about the consequences of invading a hostile Arab country. Plenty of voices -- all over the world -- accurately predicted the situation America now finds itself in. But in America, the only voices (apparently) that are allowed to be skeptical of military force are those currently wearing a uniform and in active duty. It's ridiculous how narrow this is, but look back and even prominent war skeptics who were "merely" ex-military were sidelined. Scott Ritter, (USMC, ret.) and former weapons inspector, was proven right but slimed and then ignored at the time. Hell, Lt. Gen Brent Scowcroft (USAF, ret.) came as close as possible to delivering poppa Bush's own spanking to his son, and was acknowledged but had zero impact. The war critics who didn't have the advantage of having killed people in America's name might as well have not existed in the American media.
Worrying about whether or not "the Generals" will speak up next time misses the point -- we shouldn't be relying on the Generals to speak about foreign policy ex cathedra in the first place. Rather, we need a media that allows war skeptics to be seen as legitimate and co-equal to the Generals. Until we fix that, pinning our hopes on whether or not we have competent officers at the Pentagon is a waste of time.