Stepping in a bit here:
We've had some of our mid-level generals retire then say they didn't like the mission in their post-retirement books.
While I agree that the military top brass should never disagree with the political masters in public, in private, they are paid to offer their best advice, even if contrary to what the politicians want.
Then, if they can't stomach the order to proceed, they should resign. None did.
The only one of our generals who spoke up was US Army Chief of Staff Eric Shinseki. When asked during Congressional hearings before the war, he said it would take several hundreds of thousands of troops and many years.
He successor was announced shortly thereafter. Shinseki found out about that at the same time the press did and then SECDEF did not attend his retirement ceremony. Very shoddy treatment, in my opinion.
At, to my knowledge, Shinseki still hasn't made a public statement in retirement about "I told you so." A true gentleman.