The residence of the landgraves was in Darmstadt, hence the name. As a result of the Napoleonic Wars, the landgraviate was elevated to the Grand Duchy of Hesse following the Empire's dissolution in 1806.
The Hesse-Rheinfels line became extinct on Philip's death in 1583. When, in 1604, the childless Landgrave Louis IV of Hesse-Marburg died at Marburg Castle, a succession dispute to his lands, along with the sectarian differences between Calvinist Hesse-Kassel and Lutheran Hesse-Darmstadt, led to a bitter, decades-long rivalry. Because the University of Marburg had become Calvinist under the rule of Landgrave Maurice of Hesse-Kassel, his cousin Louis V of Hesse-Darmstadt founded the Lutheran University of Giessen in 1607.
The inheritance conflict was continued in the broader context of the Thirty Years' War, in which Hesse-Kassel sided with the Protestant estates and Hesse-Darmstadt sided with the Habsburg emperor. The Hesse-Homburg and Hesse-Rotenburg estates seceded from the opponents in 1622 and 1627. Though Hesse-Darmstadt and Hesse-Kassel reached an agreement in 1627, the quarrels rekindled, resulting inter alia in the Siege of Dorsten and culminating in a series of open battles from 1645, when the Kassel Landgravine Amalie Elisabeth besieged Marburg. The conflict was finally settled on the eve of the 1648 Peace of Westphalia, more than eighty years after the division of the estates. Large parts of the disputed Upper Hesse territory, including Marburg, fell to the elder Kassel line, while Hesse-Darmstadt retained Giessen and Biedenkopf.
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