Identifying dead soldiers on the battlefield has been a problem nearly since the birth of war itself. Unless there were men alive who still knew a soldier or who were eyewitnesses to his death, soldiers were typically not returned to their homes and simply buried in mass graves.
During the Civil War armies on both sides tried various methods of identifying individuals should they be killed. They often carried photographs into battle with names of relatives inscribed, or had sewn strips of cloth into their coats with their name, regiment and company, and possibly hometown. Some Confederate men cleverly engraved pieces of bone or ivory with their designations, and others even carved them on acorns.
But the generally held belief is that the armies of America did not standardize an official way to identify soldiers until World War I, half a century after the War Between the States.
This has been proven false.
We now know that while there was no standard throughout the army, certain regiments did more to ensure the safety of their men than others.
Archeological excavators in Delaware and in Gettysburg have unearthed these unusual copper or bronze coin-shaped medallions clearly meant to be worn around the neck. On the front we see the soldier's name, Regiment and Company engraved in a circle. On the back is the Union battle eagle with WAR 1861, UNITED STATES.
This particular tag or coin or disc belonged to Andrew Howard, of the 2nd Delaware Volunteers, Company H. Several of these medallions are in possession of members of my reenacting group.
They can be found but are apparently very rare. I do not know if this was a standard in any other part of the Union Army, or if it's unique to my State's militia in 1861.
So if anyone says dog tags weren't around until the First World War you can say that is incorrect. :)