SIDNEY or (SYDNEY), ALGERNON (1622-1683), Eng lish politician, second son of Robert, 2nd earl of Leicester, and of Dorothy Percy, daughter of Henry, 9th earl of Northumberland, was born at Penshurst, Kent. As a boy he showed much talent, which was carefully trained under his father's eye. In 1632 with his elder brother Philip he accompanied his father on his mission as ambassador extraordinary to Christian IV. of Denmark. In May 1636 Sidney went with his father to Paris, where he became a general favourite, and from there to Rome. In Oct. 1641 he was given a troop in his father's regiment in Ireland, of which his brother, known as Lord Lisle, was in command. In Aug. 1643 the brothers returned to England. At Chester their horses were taken by the Royalists, whereupon they again put out to sea and landed at Liverpool. Here they were detained by the Parlia mentary commissioners, and by them sent up to London for safe custody. From this time Sidney ardently attached himself to the Parliamentary cause. On May 1o, 1644, he was made captain of horse in Manchester's army, under the Eastern Association. He was shortly afterwards made lieutenant-colonel, and charged at the head of his regiment at Marston Moor (July 2), where he was wounded, and rescued with difficulty. In April 1645 he was given the command of a cavalry regiment in Cromwell's division of Fairfax's army, was appointed governor of Chichester on May 1o, and in December was returned to parliament for Cardiff. In July 1646 he went to Ireland, where his brother was lord lieutenant, and was made lieutenant-general of horse and gov ernor of Dublin. He had hardly reached Ireland when he was recalled and stationed at Dover castle as governor. He was at this time acting with the Independents, and was nominated one of the commissioners for the trial of Charles I. But he took no part in the trial, and remained at Penshurst until the trial was over. He states that he opposed the proceedings on the grounds, "first, that the king could be tried by no court; second, that no man could be tried by that court." In 1651 he lost his governor ship of Dover castle, and spent some months in Holland. In the autumn he became a member of the council of State, but he dis approved of Cromwell's assumption of the supreme power, and retired to Penshurst, and then to The Hague, where he became a close friend of De Witt.
Upon the restoration of the Long Parliament, in May 1659, Sidney again took his seat, and was placed on the council of State. He was now, as before, especially concerned with the foreign work of the council. In June he was appointed one of three commissioners to mediate for a peace between Denmark, supported by Holland, and Sweden. He was probably intended to watch the conduct of his colleague, Admiral Montagu (after wards 1st earl of Sandwich), who was in command of the Baltic squadron. Upon the conclusion of the treaty he went to Stock holm as plenipotentiary; and in both capacities he behaved with resolution and address. Meanwhile the Restoration had taken place and Sidney, instead of returning to England, went to Copen hagen, and then to Hamburg to await the turn of events. He then travelled by way of Venice to Rome. His father sent him very little money. Five shillings a day, he says, served him and two men very well for meat, drink and firing in Rome. He de voted himself to the study of books, birds and trees, and speaks of his natural delight in solitude being largely increased. In 1663 he left Italy, passed through Switzerland, where he visited Lud low, and came to Brussels in September, where his portrait was painted by van Egmondt ; it is now at Penshurst. He had thoughts of joining the imperial service, and offered to transport from England a body of the old Commonwealth men; but this was refused by the English court. The enmity against him was so great that now, as on other occasions, attempts were made to assassinate him. On the breaking out of the Dutch war, Sidney, who was at The Hague, urged an invasion of England, and shortly afterwards went to Paris, where he offered to raise a rebellion in England on receipt of ioo,000 crowns. Unable, however, to come to terms with the French Government, he once more went into retirement in 1666,—this time to the south of France. In Aug. 167o he was again in Paris, and Arlington proposed that he should receive a pension from Louis; Charles II. agreed, but insisted that Sidney should return to Languedoc.