I agree with Bob about the need for moral (and I'd add intellectual) courage.

My thought is that an unquestioningly obedient general officer corps undermines the democratic institution of the country. If the military is only responsible to the executive (and narrowly only accountable for the execution of predetermined policy), then that leaves no room for conversation about the most effective means when the executive more or less has the power to pursue military operations without Congressional input. DoD needs to adopt the slogan "See something, say something" into its values system. Even if the goals were unattainable, that does not excuse the senior military leadership from accepting it without question.