Alex Garg writes...

"Although I don't know if they actually used the M.I.A. acronym as far back as WWII. I associate it with Vietnam. Does anyone else know?"

The farthest back I've seen militaries use "Missing," not necessarily "M.I.A.," on casualty lists is the Crimean War. I know the U.S. used "Missing" during the Civil War. Before then, armies had "Unclassified" casualties because it was nearly impossible to tell if someone was missing as a result of a battle, was mixed up with another unit or had gotten scared and ran from the battle.

But going back to your actual question, the acronym came about during WWI (or at least that's when the U.S. began keeping track of M.I.A. figures) and was very much used in WWII. The U.S. Department of Defense Defense Prisoner of War/Missing Personnel Office's mission of recovering M.I.A.'s begins with those missing from WWII.

Probably the reason why you associate the acronym with Vietnam is because the U.S. added the acronym M.I.A.P.D. - Missing in Action Presumed Dead - to its acronym-heavy lexicon either shortly before or during Vietnam, and because the government didn't want to keep reporting PD's to the media, they more readily reported those who were M.I.A. and might be found alive (of course, they might have been reporting PD's as well and just never informed the general public about the acronym's extension).

Sobering statistic time: Of the 217,000 U.S. soldiers reported M.I.A. from WWI through Vietnam, 42% remain unaccounted for; 88,000 of those still missing are from WWII-Vietnam.

Anyway, that's the best I can do with that - maybe someone else knows more. Thanks for the ramble, I hope you have more on the way.