Gravel seems to be the only presidential candidate willing to admit the obvious truth that it's possible for American soldiers to die in vain: "There's only one thing worse than a soldier dying in vain; it's more soldiers dying in vain." Obama, disappointingly, parrots the silly dogma demanded of him by the American public: "I never think that troops... who do their mission for their country, are dying in vain."
What does that mean, exactly? The nationalist dogma seems to imply that it's always worthwhile for your soldiers to die. But why would anyone want their politicians to believe that? Gravel's position seems to show deeper support for the troops, as he recognizes the moral duty of a commander in chief to not throw their lives away. The other politicians, by contrast, all piously profess that it's impossible to throw away the lives of soldiers; whatever the commander tells them to do is ipso facto glorious and worthwhile. (Dulce et decorum est pro patria mori.) They thus trivialize the moral burden of leadership. It's easy to see why a failed leader might want to do this; but why in the world would the broader public fall for it -- and all in the name of "supporting the troops", no less!?
The problem seems to stem from the sort of insecure patriotism that's so unwilling to admit mistakes that it forsakes any opportunity for genuine improvement. (Such self-idolizing "patriots" are arguably the biggest obstacle to true American greatness.)
It may also be an unfortunate encroachment of non-cognitive discourse. That is, the public don't care about the literal truth of whether troops are dying in vain. It's just a tribal signal. Bad, anti-American people claim that the troops are dying in vain. So, whether it's true or not, public figures must deny the claim merely to distance themselves from the bad tribe. (Cf. their public professions of religious "faith". Only evil communists are atheists, after all!)
Now, to create a society where politics was rational and truth-oriented... that would be worth dying for.