The Hapsburg lands were reunited under Maximilian I at the end of the 15th cent. In the meantime, Tyrol (1363), NE Istria (1374), and Trieste (1382) were added to the Hapsburg domain. Albert V of Austria, married to a daughter of Holy Roman Emperor Sigismund, succeeded him as king of Bohemia and Hungary and was chosen (1438) German king as Albert II. Henceforth, with one exception, the head of the house of Hapsburg was elected German king and Holy Roman emperor (see Holy Roman Empire for a complete list of emperors).
Though Holy Roman Emperor Frederick III raised (1453) Austria to an archduchy and acquired (1471) Fiume, he had to struggle to maintain the Hapsburg realms during his constant warfare against Matthias Corvinus, king of Hungary and Bohemia. Under Frederick and his son, Maximilian I, a series of marriages greatly increased the hereditary holdings of the dynasty and gave rise to the saying "Let others wage war; thou, happy Austria, marry."
Most of the Low Countries (see Netherlands, Austrian and Spanish) were acquired by the marriage of Maximilian to Mary of Burgundy. The marriage of their son, Philip I, to Joanna of Castile, brought Philip's elder son, Holy Roman Emperor Charles V, to the throne of Spain. The marriage of Charles's younger brother, Ferdinand, to Anna, daughter of Louis II of Bohemia and Hungary, strengthened the Hapsburg claim to these possessions after the death (1526) of Louis at Mohács. Hapsburg power reached its zenith under Charles V.
The reigns of Maximilian I and Charles V, while encompassing the height of Hapsburg power, also witnessed the emergence of the enduring struggles that eventually sapped Hapsburg strength. These included the defense of Central Europe against the Turks; the support of the Catholic Church against the Protestant Reformation; and the defense of the dynastic position against the rise of France.
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