Johann Josef Cajetan Graf von Klenau, Freiherr von Janowitz (Czech: Jan hrabě z Klenové, svobodný pán z Janovic; 13 April 1758 – 6 October 1819) was a field marshal in the Habsburg army. Klenau, the son of a Bohemian noble, joined the Habsburg military as a teenager and fought in the War of Bavarian Succession against Prussia, Austria's wars with the Ottoman Empire, the French Revolutionary Wars, and the Napoleonic Wars, in which he commanded a corps in several important battles.

In the early years of the French Revolutionary Wars, Klenau distinguished himself at the Wissembourg lines, and led a battle-winning charge at Handschuhsheim in 1795. As commander of the Coalition's left flank in the Adige campaign in northern Italy in 1799, he was instrumental in isolating the French-held fortresses on the Po River by organizing and supporting a peasant uprising in the countryside. Afterward, Klenau became the youngest lieutenant field marshal in the history of the Habsburg military.

As a corps commander, Klenau led key elements of the Austrian army in its victory at Aspern-Essling and its defeat at Wagram, where his troops covered the retreat of the main Austrian force. He commanded the IV Corps at the 1813 Battle of Dresden and again at the Battle of Nations at Leipzig, where he prevented the French from outflanking the main Austrian force on the first day of the engagement. After the Battle of Nations, Klenau organized and implemented the successful Dresden blockade and negotiated the French capitulation there. In the 1814–15 campaign, he commanded the Corps Klenau of the Army of Italy. After the war in 1815, Klenau was appointed commanding general in Moravia and Silesia. He died in 1819.

Johann Josef Cajetan von Klenau und Janowitz was born into an old Bohemian nobile family at Benatek Castle in the Habsburg province of Bohemia on 13 April 1758. The family of Klenau dates to the fifteenth century, and the family of Janowitz to the fourteenth. The family name of Klenau regularly appears in records after the sixteenth century. The Klenau family was one of the oldest dynasties in Bohemia, and many of the noble families of Bohemia have sprung from marriages into the Klenau line. The original name of the family was Przibik, with the predicate von Klenowa. The family was raised to the baronetcy in 1623 with the certificate granted to one Johann von Klenowa and, in 1629, to his son, Wilhelm. The Imperial councilor and judge in Regensburg, Wilhelm von Klenau, was raised to comital status in 1630, and to the status of Reichsgraf, or imperial count, in 1633.

Johann Klenau entered the 47th Infantry Regiment Ellrichshausen in 1774, at the age of 17, and became a second lieutenant in 1775. After transferring to a Chevauleger regiment as a Rittmeister, or captain of cavalry, Klenau fought in the short War of the Bavarian Succession, also known as the Potato War. Most of this conflict occurred in Bohemia (part of the modern Czech Republic) from 1778 to 1779, between the Habsburg monarchy, Saxony, Bavaria and Prussia. The war had no battles, but was instead a series of skirmishes and raids, making it the ideal situation for a captain of light cavalry. In their raids, forces from both sides sought to confiscate or destroy the other's provisions, fodder, and materiel.

In the Austro–Turkish War (1787–1791), one of the House of Habsburg's many 18th-century wars with the Ottoman Empire, Klenau served in the 26th Dragoon Regiment Toscana, and later transferred to the 1st Dragoon Regiment Kaiser. His regiment repulsed an attack of superior numbers of Ottoman forces on 28 September 1788, at Zemun, near Belgrade, for which he received a personal commendation and earned his promotion to major. In his early military career Klenau demonstrated, not only at Zemun but also in the earlier skirmishing and raids of 1778 and 1779, the attributes required of a successful cavalry officer: the military acumen to evaluate a situation, the flexibility to adjust his plans on a moment's notice, and the personal courage to take the same risks he demanded of his men.

Initially, the rulers of Europe considered the 1789 revolution in France as an affair between the French king and his subjects, and not a matter in which they should interfere. However, as the rhetoric grew more strident after 1790, the European monarchs began to view the French upheavals with alarm. Among the concerned monarchs were the Holy Roman Emperor Leopold II, who feared for the life and well-being of his sister, the Queen of France, Marie Antoinette. In August 1791, in consultation with French émigré nobles and Frederick William II of Prussia, he issued the Declaration of Pillnitz, in which they declared the interest of the monarchs of Europe as one with the interests of Louis XVI and his family. They threatened ambiguous, but quite serious, consequences if anything should happen to the royal family.

The French Republican position became increasingly difficult. Compounding problems in international relations, French émigrés agitated for support of a counter-revolution. From their base in Koblenz, adjacent to the French–German border, they sought direct support for military intervention from the royal houses of Europe, and raised an army. On 20 April 1792, the French National Convention declared war on Austria and its allies. In this War of the First Coalition (1792–1798), France ranged itself against most of the European states sharing land or water borders with her. Portugal and the Ottoman Empire also joined the alliance against France.