In WWII, Allied forces used the serial number information on German tanks to (correctly) extrapolate the Nazis rate of tank production.
During World War I, German and Allied forces came out of their trenches, on XMas 1914, to sing carols, trade equipment and play games between their lines.
The only recorded air-to-air helicopter battles occurred in the Iran-Iraq war between Iraq Mil-25 and Iranian AH-1J.
The US flew nearly $12bn in shrink-wrapped $100 bills into Iraq, then distributed the cash with no proper control over who was receiving it and how it was being spent.
The American Civil War is re-enacted in other countries.
During WWII a US congressman gave president Roosevelt a letter opener made from a Japanese soldier’s arm.
There was special unit of the soviet police that investigated and battled acts of cannibalism during the Battle of Leningrad.
About as many horses were killed on the Western Front in WWI as people (8 million).
The Nazis were responsible for a European raccoon population explosion.