There are different species of treaties according to their different objects: treaties of alliance; treaties of boundaries; treaties of cession; treaties of navigation and commerce; treaties of neutrality; treaties of peace.
121. Essential conditions of public treaties.—As a principle, the rules which govern international compacts are (with the exception of a few differences) the same as those which govern private compacts. There are three fundamental conditions: 1, the consent; 2, a licit cause; 3, the capacity of the contracting parties. (See above, 92.)
The consent should be: 1, declared; 2, free; 3, mutual.
The licit causes are those which are physically possible or morally legitimate; the illicit causes are those which are contrary to morality, as, for example, would be the establishment of slavery.
The capacity of making a compact belongs to the sovereign of the State alone; but it is necessary that this sovereign be really invested with the power. A sovereign stripped of his sovereignty has no power to make compacts, although he might have all the most legitimate rights; and, on the other hand, a usurping power can legitimately make compacts. The reason of this is, that foreign nations are not capable to decide what with another people constitutes the legitimacy or non-legitimacy of power: there is for them, therefore, only the power de facto. Yet this is but the general rule. There may be cases where a foreign government may refuse to recognize a usurper’s power.
122. Observance of treaties.—The obligation to observe treaties is based on the natural law. Whether compacts take place between States or individuals, it matters little. The States, in respect to each other, are like private individuals. Certain publicists, particularly Machiavelli, have maintained that the obligation to observe treaties only lasts as long as these accord with our interests. As much as to say that one should not make any compacts. Besides, Machiavelli’s opinion is in such disrepute that it is almost useless to discuss it. We will content ourselves with setting against it the following beautiful thought of a great politician:
Kings should be very careful in making treaties, but when once made, they must observe them religiously. I know very well that many politicians teach the contrary; but without stopping to consider what Christianity has to say regarding these maxims, I maintain that, since the loss of honor is greater than that of life, a great prince should rather risk his person, and even the loss of his State, than break his word, which he cannot break without losing his reputation, consequently, his greatest strength as a sovereign. (Cardinal de Richelieu, Testament politique, 2e partie, ch. vi.)