Philip The Magnanimous, landgrave of Hesse, born in Marburg, Nov. 13,1504, died in Cassel, March 31,1567. In 1509 he succeeded his father William II., under the regency of his mother, and in 1523 married a daughter of George the Bearded, duke of Saxony. He was one of the earliest champions of the reformation, and in 1531, with John the Constant, the elector of Saxony, and other members of the empire, formed the Smalcald league, and subsequently shared its military leadership with the elector John Frederick. Maurice of Saxony, who married his daughter in 1541, and who nevertheless cooperated with the emperor Charles V. in the battle of Miihlberg (April 24, 1546), in which the league was overwhelmingly defeated, subsequently turned against the emperor and insisted upon the release of Philip, his father-in-law, who had been treacherously retained a captive, as a condition of the treaty of peace at Passau (Aug. 2, 1552); but Philip was compelled by the emperor to pay a large indemnity. He afterward resumed the government of his landgraviate, and bequeathed Cassel, Marburg, Rheinfels, and-Darmstadt respectively to his four sons.
During the lifetime of his wife, who was a Catholic, he had secretly married in 1540 a Protestant lady, Margaret von der Saale. The assent of Luther and Melanchthon to this act, on his plea that the former marriage was adulterous, occasioned great scandal. Among his biographers are Rommel (Giessen, 1830), Hofmeister (Cassel, 1846), and Rinck (Darmstadt, 1852).