De iure belli ac pacis (English: On the Law of War and Peace) is a 1625 book written by Hugo Grotius on the legal status of war that is regarded as a foundational work in international law.^[1][2][3][4] The work takes up Alberico Gentili's De jure belli of 1598,^[5] as demonstrated by Thomas Erskine Holland.^[6] The book was written in Latin and published in Paris.

Its content owed much to Spanish theologians of the previous century, particularly Francisco de Vitoria and Francisco Suárez, working in the Catholic tradition of natural law.^[7]

Grotius began writing the work while in prison in the Netherlands. He completed it in 1623, at Senlis, in the company of Dirck Graswinckel.^[8]

According to Pieter Geyl:

It is an attempt by a theologically and classically educated jurist to base upon law order and security in the community of states as well as in the national society in which he had grown up. In the rather naïve rationalism, the belief in reason as the lord of life, is revealed the spiritual son of Erasmus.^[9]

In particular, this work is remembered for the sentence:

Et haec quidem quae iam diximus, locum aliquem haberent etiamsi daremus, quod sine summo scelere dari nequit, non esse Deum, aut non curari ab eo negotia humana.^[10]
What we have been saying would have a degree of validity even if we should concede that which cannot be conceded without the utmost wickedness: that there is no God, or that the affairs of men are of no concern to Him.^[11]

Such a concept has been synthesized with the famous Latin phrase etsi Deus non daretur,^[12][13] which means "even when God were assumed not to exist" but is normally translated "as if God did not exist".