For the student of International Law the treatise of Grotius, _De Jure belli et pacis_, still remains the text-book on which the later superstructure has been reared.

His _Mare liberum_, written expressly to controvert the Portuguese claim of an exclusive right to trade and navigate in the Indian Ocean, excited much attention in Europe, and was taken by James I to be an attack on the oft-asserted _dominium maris_ of the English crown in the narrow seas.
It led the king to issue a proclamation forbidding foreigners to fish in British waters (May, 1609).

Selden's _Mare clausum_ was a reply, written by the king's command, to the _Mare liberum_.

Of his strictly historical works the _Annales et Historiae de Rebus Belgicis_, for its impartiality and general accuracy no less than for its finished and lucid style, stands out as the best of all contemporary accounts from the Dutch side of the Revolt of the Netherlands.

As a theologian Grotius occupied a high rank.