England, France, Portugal, Germany, Russia, the United States all used the Law of Nations. (Spain not so much so.) Any competent judge who's not an activist judge who legislates from the bench today has a copy of the "Law of Nations" next to him.
Vattel's Law of Nations, Book 1, Chapter XVlll.
§ 208. A question on this subject.
>" But it is questioned whether a nation can, by the bare act of taking possession, appropriate to itself countries which it does not really occupy, and thus engross a much greater extent of territory than it is able to people or cultivate. It is not difficult to determine that such a pretension would be an absolute infringement of the natural rights of men, and repugnant to the views of nature, which, having destined the whole earth to supply the wants of mankind in general, gives no nation a right to appropriate to itself a country, except for the purpose of making use of it, and not of hindering others from deriving advantage from it. The law of nations will, therefore, not acknowledge the property and sovereignly of a nation over any uninhabited countries, except those of which it has really taken actual possession, in which it has formed settlements, or of which it makes actual use. in effect, when navigators have met with desert countries in which those of other nations had, in their transient visits, erected some monument to show their having taken possession of them, they have paid as little regard to that empty ceremony as to the regulation of the popes, who divided a great part of the world between the crowns of Castile and Portugal.1
There is another celebrated question, to which the discovery of the New World has principally given rise. It is asked whether a nation may lawfully take possession of some part of a vast country, in which there are none but eratic nations whose scanty population is incapable of occupying the whole? We have already observed (§ 81), in establishing the obligation to cultivate the earth, that those nations cannot exclusively appropriate to themselves more land than they have occasion for, or more than they are able to settle and cultivate. Their unsettled habitation in those immense regions cannot be accounted a true and legal possession; and the people of Europe, too closely pent up at home, finding land of which the savages stood in no particular need, and of which they made no actual and constant use, were lawfully entitled to take possession of it, and settle it with colonies. The earth, as we have already observed, belongs to mankind in general, and was designed to furnish them with subsistence: if each nation had, from the beginning, resolved to appropriate to itself a vast country, that the people might live only by hunting, fishing, and wild fruits, our globe would not be sufficient to maintain a tenth part of its present inhabitants. We do not, therefore, deviate from the views of nature, in confining the Indians within narrower limits, However, we cannot help praising the moderation of the English Puritans who first settled in New England; who, notwithstanding their being furnished with a charter from their sovereign, purchased of the Indians the land of which they intended to take possession.2 This laudable example was followed by William Penn, and the colony of Quakers that he conducted to Pennsylvania..."<
Vattel: The Law of Nations: Book I
Give a man a fish and he eats for a day. Teach a man to fish and he stops voting for the Free Fish party.