In 1813 he negotiated a treaty for mutual defence with Austria; but after the Battle of Lützen (1813), the king of Saxony returned to the French fold at Dresden. Langenau resigned and fought for Austria. On 27 July 1813, Langenau was Major General to the General Staff. At Dresden, 26 August 1813, he was successful taking command from the wounded Major General v. Frierenberger. In Leipzig, 16 October 1813, Feldm. Director-Lieut. Reisner gave direction to Langerau of the Center and the left wing batteries, united at two points, and silenced the enemy guns. On 18 October Langerau was the first to pursue the withdrawal of the enemy from the Wachau Valley. Langerau commanded artillery in battles near Hochheim (on 9 November 1813). His service as Quartermaster-General of the Grand Army of Bohemia, in 1814, and of the Rhine Army, in 1815, under Schwarzenberg, won him universal recognition. Joseph Radetzky von Radetz served under him in 1814.