Married in 1609 to Marie van Reigersberg, whose devotion was worthy of his deep affection, and loaded with public honors, having been named the official historian of the United Provinces and the advocate-general of two provinces, Holland and Zeeland, Grotius set his hand to a work entitled "_Mare Librum_," in which he defended the freedom of the sea and the maritime rights of his country against the arrogant pretensions of the Portuguese in suppressing the commerce of other nations in Eastern waters,--a treatise destined to become still more celebrated in the history of international law by Selden's reply, "_Mare Clausum_," written in 1635. Next, turning his attention to the history of The Netherlands, he devoted himself for a time to his "Annals of the War of Independence." In 1613, Grotius added to his laurels as poet, jurist, and historian by entering the field of politics, and he was appointed Pensionary of Rotterdam upon the condition that he should continue in office during his own pleasure. It was during a visit to England upon a diplomatic mission in this same year that he met the great scholar Isaac Casaubon, who said in a letter to Daniel Heinsius: "I cannot say how happy I esteem myself in having seen so much of one so truly great as Grotius. A wonderful man! This I knew him to be before I had seen him; but the rare excellence of that divine genius no one can sufficiently feel who does not see his face and hear him speak. Probity is stamped on all his features." Closely related by personal friendship as well as by his official duties to the Grand Pensionary, John of Oldenbarneveld, Grotius was destined to share with that unfortunate patriot the proscription and punishment which Maurice of Orange visited upon the two confederates in the defense of religious tolerance. Risking all as the apostles of peace, they were soon condemned to be its martyrs. Oldenbarneveld, having incurred the bitter hatred of the Stadtholder, was condemned to death by decapitation on May 12th, 1619. Grotius, less offensive to Maurice on account of his youth and his gracious personality, was sentenced six days later to perpetual imprisonment. On the 6th of June, 1619, he was incarcerated in the fortress of Loevestein.