Over There: Miniatures Rules for warfare 1900 to 1929 (2000)

From the rules:

A tactical study of warfare in the era of the First World War. OVER THERE allows the player to fight every aspect of land combat during the first two decades of the 20th Century. Poison gas, machine guns, aircraft, tanks, trenches and barbed wire are all accounted for in this accurate but easily learned format. Organizations for all major and many minor participants are given to allow you to re-fight actions all over the globe.

"A very playable... set of rules and well worth dipping into." (Miniature Wargames)

Courtesy of the author, Nick Yankosky, the rules are now available for free at http://www.freewargamesrules.co.uk/Rules/World_War_1/OverThere.pdf