I am willing to hear you out on this, but the Pentagon employs a lot of personnel, can you demonstrate that the sentiment you describe was actually representative?

Clearly those who do believe in this intervention don't have the same incentive to speak up as those that disagree with it.

It is also rather vague to conflate warnings with disagreement:

They can believe in the validity of an approach but still have the legal obligation to not just inform the president of the values and benefits of such a mission, but also warn him of any potential negative outcomes.

Warning someone about a path of action, is not equivalent to disagreeing with that path of action, it can be their job description to provide such warnings.

That said, I would like to read more about what you are referring to, to make sure we are talking about the same things.