A Yorkshire pensioner is trying to find anyone who knows the rules of Militaire, a card game that was played in the trenches of World War I.
The York Press reports that Brian Elsegood believes a deck of cards he owns originally belonged to his uncle, private Arthur Elsegood of 4th Battalion, Northumberland Fusiliers, who died in July 1918 at the age of 24.
The cards all have military ranks, and the game seems to have involved NCOs (such as lance corporals, corporals and sergeants) against officers (including second lieutenants, colonels and brigadiers).
The two different groups of cards are marked with the appropriate insignia of rank: stripes for the NCOs, epaulette badges for the officers.
Elsegood has attempted to find out how to play the game, but so far with no success. He has discovered, however, that it was played by soldiers during World War I.
First World War Centenary, 1914-1918 
A simple google search shows lots of results; with packs (with rules) regularly for sale on eBay etc.
Militaire is a card game for three or four players.N.B. Two packs should be used for five or more players. Pack consists of 54 cards representing the all the ranks of the British Army.
Object of the game is to make up sets of three or four cards of a kind, or a run of 3, 4, 6 or 7 cards in order or rank.
Thanks for the comment. Internet does seem to have some answers and Imperial War Museum has an image of a pack and some cards on its site.