He fought to uphold Protestantism against the Catholic Charles V, Holy Roman Emperor. That he engaged in bigamy and forced Luther and Melanchthon to condone this was one of the main events that weakened Protestantism during its early days. After a lost battle, Philipp was caught by the Imperial troops and jailed; he was only freed years later after some concessions and due to pressure of other Protestant princes.
On his death, his territories were divided (Hesse becoming Hesse-Kassel, Hesse-Marburg, Hesse-Rheinfels, and Hesse-Darmstadt) between his four sons by his first wife, Catherine of Saxony (daughter of George, Duke of Saxony), namely Wilhelm IV von Hessen-Kassel, Ludwig IV (III) von Hessen-Marburg, Philipp II von Hessen-Rheinfels, and Georg I von Hessen-Darmstadt.
Philipp was by all contemporary descriptions a highly intelligent and gifted but also particularly haughty and selfish person; the epithet "magnanimous" thus surprises. However, it seems now that this, the translation of der Großmütige, is actually a misinterpretation; while großmütig means indeed magnanimous in modern German, in Renaissance German, it appears to indeed have meant haughty.
Philipp I, Landgraf von Hessen, "the Magnanimous" (