The lyrics to When Johnny Comes Marching Home were written by the Irish-American bandleader Patrick Gilmore during the American Civil War. Its first sheet music publication was deposited in the Library of Congress on September 26, 1863, with words and music credited to "Louis Lambert"; copyright was retained by the publisher, Henry Tolman & Co., of Boston.[1] Why Gilmore chose to publish under a pseudonym is not clear, but popular composers of the period often employed pseudonyms to add a touch of romantic mystery to their compositions.[2] Gilmore is said to have written the song for his sister Annie as she prayed for the safe return of her fiancé, Union Light Artillery Captain John O'Rourke, from the Civil War,[3][4] although it is not clear if the engagement already existed in 1863, as the two were not married until 1875.[5]

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When Johnny Comes Marching Home was immensely popular and was sung by both sides of the American Civil War.[12]

http://en.wikipedia.org/wiki/When_Johnny_Comes_Marching_Home