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De Jure Belli ac Pacis

Living in the times of the Eighty Years' War between Spain and the Netherlands and the Thirty Years' War between Catholic and Protestant European nations (Catholic France being in the otherwise Protestant camp), it is not surprising that Grotius was deeply concerned with matters of conflicts between nations and religions. His most lasting work, begun in prison and published during his exile in Paris, was a monumental effort to restrain such conflicts on the basis of a broad moral consensus. Grotius wrote:

Fully 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 the subject. Throughout the Christian world I observed a lack of restraint in relation to war, such as even barbarous races should be ashamed of; I observed that men rush to arms for slight causes, or no cause at all, and that when arms have once been taken up there is no longer any respect for law, divine or human; it is as if, in accordance with a general decree, frenzy had openly been let loose for the committing of all crimes.

De jure belli ac pacis libri tres (On the Law of War and Peace: Three books) was first published in 1625, dedicated to Grotius' current patron, Louis XIII. The treatise advances a system of principles of natural law, which are held to be binding on all people and nations regardless of local custom. The work is divided into three books: