The 12th Army was an field army-level command of the Austro-Hungarian Army that existed only for one month during World War I.
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The Austro-Hungarian 12th Army was formed on the Eastern Front on 4 July 1916, and its commander was Archduke Karl Franz Joseph, the future Emperor of Austria. The 12th Army was again disbanded on 13 August 1916, around the time when the Archduke became commander of the Army Group Archduke Karl, fighting against the Russian Empire and Romania.
References
12th Army (Austria-Hungary) Wikipedia(Text) CC BY-SA