Mare Liberum was first printed in 1609. In it the Dutch jurist and philosopher Hugo Grotius asserted the "freedom of the seas"—a new principle that the sea was international territory.

Grotius' masterpiece De Jure Belli ac Pacis (On the Law of War and Peace) was first printed in 1625 and is the foundation of modern international law. Influenced by violence in his home country of Holland and in Europe, particularly the bloodshed of the Thirty Years' War, Grotius constructed a theory of law that advocated the restraint of armed conflict and the regulation of war. His principle of an immutable law flowing from man's inherent nature was the first attempt to regulate society and government outside church doctrine or scripture. In one passage of De Jure Belli ac Pacis, Grotius wrote that,

[f]ully convinced...that there is a common law among nations, which is valid alike for war and in war, I have had many and weighty reasons for undertaking to write upon this subject. Throughout the Christian world I have observed a lack of restraint in relation to war, such as even barbarous nations should be ashamed of. (Prolegomena, 28).

The Library's earliest edition of De Jure Belli ac Pacis is dated 1642.