I just spend a weekend in the north of France looking at the dozens of on-site war memorials, and it struck me that it's only recently that families of the fallen have had the ability/luxury to travel to the site of the battle to pay respects.
What (if any) examples are there of pre-20th-century on-site memorials to the fallen?
On site war memorials were quite common in ancient Greece. The Greeks would set up a tropaion on a tree dressed with the arms of the defeated army.
For a full on war memorial by today's standards, the Thebans erected the Lion of Chaeronea to commemorate their dead. No idea when it was erected but Pausanias mentions it in the second century BCE.
While it may not be as early as you were looking for, there are a number of memorials to the soldiers who fought at Gettysburg during the American Civil War. Monuments were erected by individual states in recognition of the regiments that fought there
http://en.wikipedia.org/wiki/List_of_monuments_of_the_Gettysburg_Battlefield
The monument at Waterloo (which changed the topography of the battlefield!) is 19th century.
Bunker Hill Monument in Boston is on Breeds Hill (where the Battle of Bunker Hill actually happened) and has been since the 19th century