The latter are various, as, for example, fear prompted by the wealth and power of a neighbor, an advantage not based upon right, the desire to gain better lands, the refusal of what we have earned by some good quality as such, the stupidity of the possessor, the desire of extinguishing a right lawfully acquired by the other party, if it seems rather irksome to us, and so forth. Hence it is permissible to deceive an enemy by a pretended speech, or fictitious reports, but not at all by promises or agreements. As for the force employed in war against the enemy and his property, we should distinguish between what an enemy can suffer without injustice, and what we cannot bring to bear against him, without violating humanity. For he who has declared himself our enemy, inasmuch as this involves the express threat to bring the worst of evils upon us, by that very act, so far as in him lies, gives us a free hand against himself, without restriction. Humanity, however, commands that, so far as the clash of arms permits, we do not inflict more mischief upon the enemy than defense, or the vindication of our right, and security for the future, require.
War is classified as formal and informal.
For the first it is required that it be waged on both sides by authority of him who has the supreme power, and that a declaration shall have preceded. A war not declared, or waged against private citizens, is informal. To this class belong also civil wars. The right of making war in a state belongs to him who has the supreme authority. Hence, to engage in war without permission given by the ruler, exceeds the authority of a magistrate, even in case he infers that the supreme power, if consulted, would decide to wage war here and now.