Cpl. Roy Holtz, the first American soldier to enter Germany in WW1, astride his trusty Harley. In this photo dated Nov 12, 1918 (the day after Armistice) he is actually leaving Germany, headed home. He and the bike were both Wisconsin natives. #VeteransDay 
the standard issue WW1 Harley (and the one Holtz is riding, sans sidecar) was the Model 17 F/J. 61 cubic inch motor, 15 hp, gas headlamp. Indian also produced bikes for the war. Olive drab remained Harley's main color for many years after.
In WW2, the standard issue Harley military bike was the WLA. 45 cubic inch motor, 21 horsepower, hardtail.
More recently, the US military's bikes of choice have been the Kawasaki KLR (650 cc, 45 HP), the HDT 1030m1 (585 cc 28 hp diesel) and the Zero MMX (all electric, 54 hp, 71 mile range, 0-60 in 4.4 seconds)
Indian also produced bikes for the military in WW1 and WW2. The WW1 Indian Powerplus Twin was 61 cubic inches, 16 hp, and actually outnumbered Harley. During WW2 they made the 741 and 841 Scouts.
