Grotius’s third son, Diederic, began his studies in a very promising manner.  Grotius writes to his brother William, August 16, 1630, “I am overjoyed to hear that Diederic’s progress even exceeds my hopes.  I wish he may continue.”  His grandfather John Grotius was his tutor.  When he came to be old enough to be put to some business, Grotius designed him for an Engineer.  He learned under the famous Boschius, and came afterwards, in the beginning of 1636, to see his father at Paris.  Grotius having applied to the Duke of Weymar to take Diederic into his service, he entered into that Prince’s houshold as one of his Pages, and was much respected:  the Duke soon after made him his Aid-de-Camp.  Grotius seemed to be well satisfied with his son at the time this youth had the misfortune to lose his Patron.

On hearing of the Duke of Weymar’s death Grotius immediately thought of sending Diederic to Marshal Bannier.  He wrote a letter to that General[771], October 13, 1639, in which he puts him in mind that it was his Excellency’s brother first proposed to him his entering into the Swedish service:  he afterwards makes an offer of his son, who had served two years under Boschius, chief Engineer to the Prince of Orange, and had since been several years one of the Duke of Weymar’s Pages.  After the death of that illustrious Prince, who shared with Bannier the glory of being the greatest general in Europe, he thinks he cannot do better than give him to Marshal Bannier, who was unanimously allowed to hold the first rank in the art of war.  He begs that his son may be only employed in things of which he is capable.  On the same day he wrote to Salvius, desiring him to recommend Diederic to the Marshal.

Whilst Grotius was thus employed in placing his son, Diederic entertained a design of entering into the Dutch service.  His father was highly displeased with this project; and wrote to him, and to William Grotius, that it was most improper for him to expose his life for his cruellest enemies; and that Sweden was his true country, and to that kingdom he ought to devote his life, and from it to merit and expect honours.  “If my son dislikes the activity of a military life, Grotius writes to his brother, he may find ease, and acquire honour in General Bannier’s houshold, or by exercising his profession of Engineer.  I shall always give him my assistance, and I hope I have as many friends elsewhere, as I have enemies in Holland.  If my son disgraces himself so far, as to ask favours from the Dutch, he is unworthy to call me father.  If he chuses rather to be an Ensign with you, than a Captain among others, he is mean-spirited, and forgetful of what he has been.”  Diederic had a design of writing the history of the Duke of Weymar; which project Grotius approved of, as worthy of a grateful mind.  He sent him word, that if he would set about this work whilst he was with Marshal Bannier, he would make his court by it to that General, who had it in his power to reward