During the War of the Sixth Coalition, he left Vienna in March 1813, and together with Friedrich Friesen and Friedrich Ludwig Jahn joined the Lützow Free Corps, a voluntary paramilitary association which Ludwig Adolf Wilhelm von Lützow was then forming in Breslau, Silesia. On his way from Vienna to Breslau, he noticed a Prussian border eagle in the area of Neustadt, which inspired him to write a poem "Der Grenzadler" (Border Eagle). In the midst of the most active campaigns, Körner continued to write poetry and other works. He wrote a singspiel, "Der vierjährige Posten", which was set to music by Franz Schubert in 1815, but the piece was not performed until 1869, when it was staged at the Hofoper, Dresden. It was later adapted in English as "The Outpost".