Austrian archduke, field marshal, army reformer, and military theoretician who was one of the few Allied commanders capable of defeating the French generals of the Napoleonic period. He modernized the Austrian army during the first decade of the 19th century, making it a formidable fighting force that contributed materially to Napoleon's defeat in 1813–15.

The third son of the future Holy Roman emperor Leopold II, Charles grew up in Italy. Taking part in the war against Revolutionary France beginning in 1792, he was victorious at Aldenhoven and Neerwinden in 1793 and became governor-general of the Austrian Netherlands the same year. He was appointed commander in chief of the Austrian Rhine army in 1796 and was also named field marshal general of the Holy Roman Empire. His campaign of 1796, in which he repeatedly defeated the French commanders Jean-Baptiste Jourdan and Jean-Victor-Marie Moreau and drove them back across the Rhine, distinguished him as one of Europe's best commanders.

Again commanding the Rhine front in the War of the Second Coalition against France (1798–1802), Charles defeated Jourdan and André Masséna but could not stop Moreau's advance on Vienna after the Austrian defeat at Hohenlinden (1800). During the war of 1805 Charles commanded the main Austrian army in Italy and again crushed Masséna at Caldiero, but Austrian defeats in Germany decided the struggle in Napoleon's favour.

After the Treaty of Lunéville (1801), Charles became president of the Austrian Hofkriegsrat (“Supreme War Council”) and generalissimo with wide powers. The only general who had vanquished the French, he discarded Austria's old military system and initiated a far-reaching program of reforms that included the adoption of the “nation-in-arms” principle, the utilization of French military organization and tactics, and the founding of military academies. Not yet ready but nevertheless a formidable force, the Austrian army under Charles crushed Napoleon at Aspern-Essling but was again defeated in the desperately fought Battle of Wagram in 1809.

Retiring during that year, Charles took no further part in the Napoleonic struggles. His military writings, especially his Grundsätze der Strategie erläutert durch die Darstellung des Feldzuges von 1796 in Deutschland, 3 vol. (1814; “Principles of Strategy, Explained Through the Description of the Campaign of 1796 in Germany”), exercised considerable influence on his contemporaries. In contrast to his aggressive and daring conduct of actual operations, Charles's writings emphasized caution and the importance of strategic points and were somewhat antiquated even in his own time.