Fernando Teson (PrawfsBlawg) has a very interesting post, which Juan links to below. Let me reproduce it here, so I can say a few things about it:

Let's assume, without deciding, that the following propositions are true:

1) Americans have a robust First Amendment right to criticize the government. This includes both the decision to go to war and the conduct of war.

2) The United States is facing a ferocious and determined enemy in Iraq.

3) The United States has a just cause, which means that victory by the United States is the morally preferable outcome.

4) Certain forms of speech (for example, strong demands that U.S. troops withdraw) objectively aid the enemy (say this speech emboldens the enemy, so more U.S. troops die and chances for victory are reduced), even if the speaker does not intend to do so.

If all this is true, isn't the speech in question morally objectionable, even if constitutionally permitted? Certainly, the fact that I have a legal right to say something doesn't morally justify my saying it. If telling you (frankly and truthfully) that your new haircut makes you look ridiculous will hurt your feelings, maybe I should refrain from saying it. This is why the only way I see morally to justify someone who aids the enemy with his speech is to deny assumption 3), that the United States has a just cause. In that case, the correct moral position is indeed to demand that the troops return. But if one accepts 3), then I cannot see how one can avoid the conclusion that the speaker is acting immorally.

It seems to me that all four of the assumptions are sound; in particular, I do think that certain kinds of antiwar speech do objectively aid the enemy. In Winston Churchill's words, statements that "weaken confidence in the Government" and "make the Army distrust the backing it is getting from the civil power" may prove to be "to the distress of all our friends and to the delight of all our foes." (Winston Churchill, Speech in the House of Commons (July 2, 1942).) Such assumptions of objective harm to the war effort are sometimes made too hastily, but it seems to me that they are often correct (though of course we can speak more confidently of their tendencies than of any precisely provable effects, since the effects of such statements on morale both of our forces and our enemies' are hard to accurately measure). This is often good reason for people to refrain from certain kinds of criticism of the war effort.

Nonetheless, it seems to me important to recognize two matters.

1. Speech often has multiple effects (and therefore so does silence). Much wartime speech may both embolden the enemy -- which the speaker may genuinely regret -- and help accomplish other morally worthy goals. For instance, exposing improper conduct by American soldiers or intelligence agents may both hurt the war effort (for instance, by hurting our military morale, weakening civilian support, moving some neutrals towards our enemies), which is bad, and stop, diminish, and deter such improper conduct, which is good. Likewise, one might think that victory is the morally preferable outcome, but not if the cost in U.S. soldiers' lives is too great; calling for a withdrawal of American troops may therefore have bad effects (reducing the likelihood of victory) as well as good ones (reducing the loss of U.S. soldiers' lives).

Conversely, a social norm that people ought not criticize the government during wartime, since this will hurt the war effort, may both help the war effort and at the same time help shelter improper behavior by the military, or increase needless waste of our soldiers' lives. We need to consider the aggregate of these effects; we should neither solely focus on those effects that decrease our chances of victory nor ignore those effects.

2. Much of the debate may have to do with people's disagreement about #4. Some people, for instance, believe that withdrawing U.S. troops will actually increase the likelihood of victory, because it will reduce one source of Iraqis' anger, and make them more willing to make peace. Those people may well be mistaken; but say they're sincere in their beliefs, and especially if the beliefs are reasonable, even if not persuasive to us. The speakers then basically share our goals (victory and saving U.S. troops lives'); they simply disagree with us on the empirically sound means to accomplish those goals. It's hard to see their speech as "morally objectionable" when it rests on such predictive disagreement, especially when the disagreement is reasonable.

So before we decide whether someone's wartime speech is morally objectionable -- even if we think the speech would aid the enemy -- it seems to me that we need to know a lot more about (1) what other effects the speech may have, and (2) what the speaker believes the likely effects of the speech will be.