Hugo Grotius (also known as Huig de Groot or Hugo de Groot; 10 April 1583 – 28 August 1645) worked as a jurist in the Dutch Republic. With Francisco de Vitoria and Alberico Gentili he laid the foundations for international law, based on natural law. He was also a philosopher, theologian, Christian apologist, playwright, and poet. Grotius's influence on international law is paramount, and is acknowledged by, for instance, the American Society of International Law, which since 1999 holds an annual series of Grotius Lectures.

Born in Delft during the Dutch Revolt, Hugo was the first child of Jan de Groot and Alida van Overschie. His father was a man of learning, once having studied with the eminent Justus Lipsius at Leiden, as well as of political distinction, and he groomed his son from an early age in a traditional humanist and Aristotelian education. A prodigious learner, Hugo entered the University of Leiden when he was just eleven years old. There he studied with some of the most acclaimed intellectuals in northern Europe, including Franciscus Junius, Joseph Justus Scaliger, and Rudolph Snellius. At age 14 he published his first book; a scholarly edition of the late antiquity writer Martianus Capellas work on the seven liberal arts.

In Holland, Grotius earned an appointment as advocate to The Hague in 1599 and then as official historiographer for the States of Holland in 1601. His first occasion to write systematically on issues of international justice came in 1604, when he became involved in the legal proceedings following the seizure by Dutch merchants of a Portuguese carrack and its cargo in the Strait of Singapore. The Dutch were at war with Spain and Portugal when the loaded merchant ship, Santa Catarina a Portuguese carrack, was captured by captain Jacob van Heemskerk in 1603. Heemskerk was employed with the United Amsterdam Company (part of the Dutch East India Company), and though he did not have authorization from the company or the government to initiate the use of force, many shareholders were eager to accept the riches that he brought back to them. Not only was the legality of keeping the prize questionable under Dutch statute, but a faction of shareholders (mostly Mennonite) in the Company also objected to the forceful seizure on moral grounds, and of course, the Portuguese demanded the return of their cargo. The scandal led to a public judicial hearing and a wider campaign to sway public (and international) opinion. It was in this wider contest that representatives of the Company called upon Grotius to draft a polemical defence of the seizure. The result of Grotius' efforts in 1604-1605 was a long, theory-laden treatise that he provisionally entitled De Indis (On the Indies). Grotius sought to ground his defense of the seizure in terms of the natural principles of justice. In this, he had cast a net much wider than the case at hand; his interest was in the source and ground of war's lawfulness in general. The treatise was never published in full during Grotius' lifetime, perhaps because the court ruling in favor of the Company preempted the need to garner public support. The manuscript was not made public until it was uncovered from Grotius' estate in 1864 and published under the title, De Jure Praedae (On the Right of Capture). The principles that Grotius developed there, however, laid the basis for his mature work on international justice, De jure belli ac pacis, and in fact one chapter of the earlier work did make it to the press in the form of the influential pamphlet, Mare Liberum.

In The Free Sea (Mare Liberum, published 1609) Grotius formulated the new principle that the sea was international territory and all nations were free to use it for seafaring trade. Grotius, by claiming 'free seas', provided suitable ideological justification for the Dutch breaking up of various trade monopolies through its formidable naval power (and then establishing its own monopoly). England, competing fiercely with the Dutch for domination of world trade, opposed this idea and claimed That the Dominion of the British Sea, or That Which Incompasseth the Isle of Great Britain, is, and Ever Hath Been, a Part or Appendant of the Empire of that Island. William Welwod, a Scottish jurist who was the first to formulate the laws of the sea in the English language, argued against Grotius' Mare Liberum in An Abridgement of All Sea-Lawes (1613), eliciting a response from Grotius around 1615 under the title Defensio capitis quinti Maris Liberi oppugnati a Gulielmo Welwodo ("Defense of the five free oceans, opposed by William Welwod"). In Mare clausum (1635) John Selden endeavoured to prove that the sea was in practice virtually as capable of appropriation as terrestrial territory. As conflicting claims grew out of the controversy, maritime states came to moderate their demands and base their maritime claims on the principle that it extended seawards from land. A workable formula was found by Cornelius Bynkershoek in his De dominio maris (1702), restricting maritime dominion to the actual distance within which cannon range could effectively protect it. This became universally adopted and developed into the three-mile limit.

The dispute would eventually have important economic implications. The Dutch Republic supported the idea of free trade (even though it imposed a special trade monopoly on nutmeg and cloves in the Moluccas). England adopted the Act of Navigation (1651), forbidding any goods from entering England except on English ships. The Act subsequently led to the First Anglo-Dutch War (1652 - 1654).

Aided by his continued association with van Oldenbarnevelt, Grotius made considerable advances in his political career, being retained as Oldenbarnevelt's resident advisor in 1605, Advocate General of the Fisc of Holland, Zeeland and Friesland in 1607, and then as Pensionary of Rotterdam (the equivalent of a mayoral office) in 1613. In 1608 he married Maria van Reigersbergen, with whom he would have eight children (four surviving beyond youth) and who would be invaluable in helping him and the family to weather the storm to come.

In these years a great theological controversy broke out between the followers of Jacobus Arminius, chair of theology at Leiden, and the strongly Calvinist theologian, Franciscus Gomarus. In 1610, several months after the death of their leader, those who dissented with Arminius issued a 'Remonstrance' declaring their doctrinal differences with the mainstream Reformed doctrines of salvation, most often associated with the Protestant Reformer John Calvin, but also held by most Reformed pastors and theologians throughout Europe. They had particular problems with the Belgic Confession, art. 16, on eternal election and reprobation. The Remonstrants did not reject the doctrines of election or predestination, as is often assumed, but rather redefined them so that the decisive factor in a person's salvation is not God's inscrutable decree, but the individual's faith, which God foresees from eternity. According to Arminius and the Remonstrants, God decrees to elect all who meet the condition of faith. Led by Oldenbarnevelt, the States of Holland took an official position of religious toleration towards the disputants, and Grotius was eventually asked to draft an edict to express this policy. The edict of 1613 put into practice a view that Grotius had been developing in his writings on church and state (Erastianism): that only the basic tenets necessary for undergirding civil order (e.g., the existence of God and His providence) ought to be enforced while differences on obscure theological doctrines should be left to private conscience. The edict did not have the intended effect, and hostilities flared throughout the republic. To maintain civil order, Oldenbarnevelt eventually proposed that local authorities be given the power to raise troops (the Sharp Resolution). Such a measure putatively undermined the authority of the stadtholder of the republic, Maurice of Nassau, Prince of Orange. Maurice seized the opportunity to solidify the preeminence of the Gomarists, whom he had supported, and to eliminate the nuisance he perceived in Oldenbarnevelt (the latter had previously brokered the Twelve Years' Truce with Spain in 1609 against Maurice's wishes). He had Oldenbarnevelt and Grotius arrested on 29 August 1618. Ultimately, Oldenbarnevelt was executed, and Grotius was sentenced to life imprisonment in Loevestein castle. In 1621, with the help of his wife and maidservant, Grotius managed to escape the castle in a book chest and fled to Paris. In the Netherlands today, he is mainly famous for this daring escape. Both the Rijksmuseum in Amsterdam and the museum Het Prinsenhof in Delft claim to have the original book chest in their collection.

Grotius was well received in Paris by his former acquaintances and was granted a royal pension under Louis XIII. It was here in France that Grotius completed his most famous philosophical works. While in Paris, Grotius set about rendering into Latin prose a work which he had compiled in prison, providing rudimentary yet systematic arguments for the truth of Christianity. (Showcasing Grotius' skill as a poet, the earlier Dutch version of the work, Bewijs van den waren Godsdienst (pub. 1622), was written entirely in didactic verse.) The Latin work was first published in 1627 as De veritate religionis Christianae. It was the first Protestant textbook in Christian apologetics, and was divided into six books. Part of the text dealt with the emerging questions of historical consciousness concerning the authorship and content of the canonical gospels. Other sections of the work addressed pagan religion, Judaism and Islam. What also distinguished this work in the history of Christian apologetics is its precursor role in anticipating the problems expressed in Eighteenth century Deism, and that Grotius represents the first of the practitioners of legal or juridical apologetics in the defence of Christian belief. Hugely popular, the book was translated from Latin into English, Arabic, Persian and Chinese by Edward Pococke for use in missionary work in the East and remained in print until the end of the nineteenth century.

Grotius also developed a particular view of the atonement of Christ known as the "Governmental" or "Moral government" theory. He theorized that Jesus' sacrificial death occurred in order for the Father to forgive while still maintaining his just rule over the universe. This idea, further developed by theologians such as John Miley, became the dominant view in Methodist Arminianism.

Living in the times of the Eighty Years' War between Spain and the Netherlands and theThirty Years' War between Catholic and Protestant European nations, 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. 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:

• Book I advances his conception of war and of natural justice, arguing that there are some circumstances in which war is justifiable.

• Book II identifies three 'just causes' for war: self-defense, reparation of injury, and punishment; Grotius considers a wide variety of circumstances under which these rights of war attach and when they do not.

• Book III takes up the question of what rules govern the conduct of war once it has begun; influentially, Grotius argued that all parties to war are bound by such rules, whether their cause is just or not.

The arguments of this work constitute a theory of just war. Roughly, the second book takes up questions of jus ad bellum (justice in the resort to war) and the third, questions of jus in bello (justice in the conduct of war). The way that Grotius conceived of these matters had, together with Francisco de Vitoria's De potestate civili, a profound influence on the tradition after him and on the later formulation of international law.

Many exiled Remonstrants began to return to the Netherlands after the death of Prince Maurice in 1625, but Grotius, who refused to ask for pardon since it would imply an admission of guilt, was denied repatriation despite his repeated requests. Driven out once again after attempting to return to Rotterdam in October of 1631, Grotius fled to Hamburg. In 1634 he met the opportunity to serve as Sweden's ambassador to France. The recently deceased Swedish king, Gustavus Adolphus had been an admirer of Grotius (he was said to have carried a copy of De jure belli ac pacis always in his saddle when leading his troops), and his successor's regent, Axel Oxenstierna, was keen to have Grotius in his employ. Grotius accepted the offer and took up diplomatic residence at Paris, which remained his home until he was released from his post in 1645. While departing from his last visit to Sweden, Grotius was shipwrecked on his voyage. He washed up on the shore of Rostock, ill and weather-beaten, and on August 28, 1645 he died; his body at last returned to the country of its youth, being laid to rest in the Nieuwe Kerk in Delft.

Grotius' personal motto was Ruit hora ("Time is running away"); his last words were "By understanding many things, I have accomplished nothing." Significant friends and acquaintances of his include the theologian Franciscus Junius, the poet Daniel Heinsius, the philologist Gerhard Johann Vossius, the historian Johannes Meursius, the engineer Simon Stevin, the historian Jacques Auguste de Thou, and the Arabic scholar Erpinius.