Montesquieu
 

XXVI.23 That when, by some circumstance, the political law destroys the state, one must decide by the political law that preserves it, which sometimes becomes a law of nations

When the poli­ti­cal law which has esta­bli­shed a cer­tain order of suc­ces­sion in the state beco­mes des­truc­tive of the poli­ti­cal body for which was made, there can be no doubt that ano­ther poli­ti­cal law could change that order ; and far from that same law being oppo­sed to the first, it will fun­da­men­tally be per­fectly com­pa­ti­ble with it, since they will both depend on the prin­ci­ple that the wel­fare of the peo­ple is the supreme law.1

I have said2 that a great state that had become acces­sory to ano­ther wea­ke­ned itself, and even wea­ke­ned the prin­ci­pal state. We know that it is in the state’s inte­rest to have its chief at home, for the public reve­nues to be well admi­nis­te­red, for its money not to go enrich ano­ther coun­try. It is impor­tant that he who is to govern not be imbued with foreign maxims : they are less appo­site than the ones already esta­bli­shed ; besi­des, men hold fier­cely to their laws and cus­toms ; in them lies the feli­city of each nation ; it is rare that they are chan­ged without great uphea­vals and a great effu­sion of blood, as the his­to­ries of all coun­tries show.

It fol­lows from this that if the heir of a great state is the pos­ses­sor of a great state, the for­mer can very well exclude him, because it ser­ves both sta­tes for the order of suc­ces­sion to be chan­ged. Thus does the law of Russia made early in the reign of Elisabeth very pru­dently exclude any heir who pos­ses­ses ano­ther monar­chy ; thus does the law of Portugal reject any forei­gner cal­led to the crown by right of blood.

Now if a nation can exclude, it has a for­tiori the right to com­pel a renun­cia­tion. If it fears lest a cer­tain mar­riage have conse­quen­ces that can cause it to lose its inde­pen­dence or force it to a divi­sion, it may very well com­pel the contrac­ting par­ties with their des­cen­dants to renounce all the rights they would have to it ; and he who renoun­ces, and those against whom he renoun­ces, will be all the less entit­led to com­plain because the state could have made a law exclu­ding them.

[Salus populi suprema lex is a widely-used motto taken from Cicero, De legibus (book III, part III, sub. viii).]

See above, book VIII, ch. xvii and following.