Premise 1: Either we will win the battle, or we will lose the battle.
Premise 2: If we will win the battle, then we ought to attack with a small force.
Premise 3: If we will lose the battle, then we ought to attack with a small force.
Conclusion: We ought to attack with a small force.
Here was the comment I left them:
I recommend that you reject premise 3: "If we will lose the battle, then we ought to attack with a small force."
Suppose the antecedent is true: We're going to lose the battle. There are many reasons why this might be the case. One of them is that we aren't going to bring a large enough force. Then it's the case that we ought to bring a large force including many werewolves and some hippogriffs. In this case, the ought-claim that is the consequent of the conditional -- "we ought to attack with a small force" is false. Since the antecedent is true and the consequent is false, the conditional is false.
(I get kind of overexcited when people are talking about battle and I have a chance to adopt a werewolf identity. Dennis might understand why.)
Right now I'm thinking that there's an ambiguity in how to interpret the conditional in 3: Is it a material or a counterfactual conditional? In my comment, I regard it as the material conditional. For the counterfactual conditional, you're supposed to consider all the possible worlds where we lose the battle, and figure out which are the best. The best of those are the ones where we bring a small force and lose. Maybe this explains why 3 seems so appealing. But I don't think the argument is valid if you use the counterfactual conditional all the way through -- the fact that the best winning-world and the best losing-world have us bringing small forces does not tell us what to do. Hmm, I think I'll go back to Fake Barn Country and mention this to them.