Carved wooden temporary grave marker of Lieutenant Charles R. Carville, a member of the 165th New York Volunteers who died at Port Hudson May 27, 1863, during the American Civil War. Division of Armed Forces History, Nation Museum of American History.
The first appearance of the "battlefield cross" is a matter of conjecture. It might have been during the Civil War, to signify a dead soldier to be gathered and buried during a truce called for that purpose. Soldier dead were buried in graves in temporary cemeteries near the battlefields, identified by simple wooden plaques. The configuration of the rifle pointed downward with a helmet perched on the stock was a more common sight during World War I and World War II. While the battlefield cross still acted as marker so that the Graves Registration Service personnel could remove the body for burial, it also began to serve as a memorial. Although it is called a cross, the memorial has no overt religious context.
Read more at: http://americanhistory.si.edu/blog/battlefield-cross