We don’t know how many Americans who died while serving during wartime are buried on foreign soil. Why? For one, there isn’t a single organization that tracks the burials. There is no database of every American service member buried overseas. And the reason why there is no database is because finding all of America’s war dead is a complicated process.

While some are buried in formal memorial cemeteries, others are in private cemeteries, mass graves and isolated burial sites. Old records have been lost and new burials are being discovered. Women were not always counted among the war dead. Thousands of Americans fought under foreign flags before the U.S. entered the world wars, and are buried as foreigners. Those missing in action or lost at sea further complicate tracking. And some have just been lost.

But one historian has been painstakingly counting, cataloging and mapping the locations of American war dead buried outside formal memorial cemeteries.

“In my view, who we celebrate on Memorial Day are the Americans who we know were American, were buried in American cemeteries, and we don’t think of the other Americans who were in ambulance services or Commonwealth forces, the French Foreign Legion,” said author Chris Dickon. “We’ve never included them in what we’ve thought of Memorial Day, basically I think because we haven’t known about them.”

While researching a book about one of the first frigates in the U.S. Navy, Chris Dickon stumbled on a burial site in Canada where 200 American casualties from the War of 1812 had been buried, and apparently, forgotten.