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 him. Diederic at last complied with his father's desire, and went to Marshal Bannier's camp. He was made a prisoner of war by the Bavarians in an unfortunate action near Dillingue and Memingue, in the end of the year 1643. Grotius immediately set all his friends to work to procure his son's liberty: he wrote to the famous John de Vert, and applied to the Duke of Bavaria that he would be pleased to send him back as soon as possible: and at the same time wrote to his son to come to him as soon as he should be at liberty, that they might consult together what was best to be done. Diederic sent his father the history of the unfortunate action in which he was taken; and Grotius printed it to give copies to his friends, and to send others to the Swedish Ministry. Diederic was carried to Tubingen, from thence to Ulm, and confined in a citadel between Ulm and Augsburg: he did not continue there long: immediately on receiving his father's letter, the Duke of Bavaria gave orders that Diederic might be set at liberty, after settling his ransom, which was fixed at a thousand florins. He came to Paris, and on his arrival Grotius wrote a letter of thanks to the Elector of Bavaria, telling him, that as he had but one way to express his gratitude, namely by promoting a general peace, which his Electoral Highness wished for, he would do all in his power to bring it about. He wrote to Ketner the Bavarian Minister to the same effect.