That's certainly true about total war where the nation's survival is threatened by defeat, but it's not true about the kind of limited wars was have seen since world war 2. In fact, because these wars are limited wars for the world powers that start them, but total wars for the postcolonial states in the receiving end, I think it's very correct that the public e.g. in the US continue to aggressively question the rationale for war even after it's begun in e.g. Vietnam, Iraq, and so on.

Henry Kissinger would certainly have been happier to see a US public united behind the Vietnam war effort because it would have made it easier to win in a geopolitical sense, but what about in a moral sense? The US wasn't threatened existentially by failure in Vietnam, and it's right that we didn't adopt the attitude expressed in your comment.