In practice, then, a declaration of war will never be justifiable. A defensive war even should never be undertaken unless a legitimate authority, with whom the decision rests, shall have both certainty of success and very solid proofs that the good accruing to the nation from the war will more than outweigh the untold evils which it will bring on the nation itself, and on the world in general.

It seems as though he's saying you shouldn't fight unless you have overwhelming assurance that you'll win. he may call that reason, I call it cowardice.