From the New York Review of Books, reviewing Walzer's book:
Thomas Aquinas is not much more helpful. He has three main norms for permissible war—declaration by competent authority, just cause, and proper intent. The last is defined as acting "to promote good or prevent evil"—a thing that can justify war as a tool of social engineering (e.g., to spread democracy and rebuff tyranny). It is not surprising then that Aquinas approved of the social engineering of his day, the Crusades (to spread Christianity and rebuff Muslimism)—which again is more useful to current jihad than to a secular democracy.
Aquinas doesn't consider "norms for permissible war". He does not consider the issue of declaration of war. He considers only whether a Christian prince may wage war without violating justice, and answers that he may if he has the right authority and he does it for the right end (just cause) and he does it with the proper intentio. And intentio in Scholastic Latin doesn't mean 'intent', it means 'the disposing of means to ends' - which can include intent, but a great deal more besides. He does not define 'intentio' as 'to promote good or prevent evil' - what he does is indicate that the intentio must be a disposition to the promotion of good or prevention of evil. He also explicitly says that those who are attacked must deserve to be attacked (this is essential to just cause), and the authority he discusses is only an authority of defense.
Further, Aquinas is simply considering here the bare issue of whether one can wage war without being unjust in so doing; he is not giving a checklist for whether 'a war' is just, and using him to this end would be pointless anyway.
Also I can't make heads or tails of Wills's reference, in a footnote, to the Scriptum Super Sententiis for the Crusades question. He gives "Scriptum super Sententiarum 4.32, 38", but 4 d. 32 and d. 38 aren't on the Crusades (they are on marriage). I don't normally read much of the Sentences commentary, so I could be missing out on some really obvious citation form, but I don't know what he's talking about in this case, and the least one could do in the New York Review of Books, when backing up your claims, is to make the citation clear. (I also have never seen the designation 'Scriptum super Sententiarum', rather than 'Sententiis'.) Actually, I can't find any questions on the Crusades (or war, or papal authority to authorize war) in the commentary, although I admit that I have not rigorously searched - there aren't any where one would expect them. Wills also gives references to the second and fifth Quodlibetal Questions. Aquinas does discuss the crucesignatus or Crusader in these questions: the first has to do with the indulgences given to Crusaders; QQ V.7.2, which asks, roughly, whether the Crusader who falls in battle has a better death than the one who dies on the way there, which does indeed assume that the Crusader is just in his vow, but does not explain further.
