  The British attempted to motorize its entire force, but the Depression stalled that attempt. The USA - indeed - achieved 99 44/100% Automotive Mobility between 1939 and 1942, and the only reason they did not achieve 100% were the facts that horses and mules were needed in China, Sicily, and Italy, where motor vehicles could not go. Even now, the US military is NOT 100% Automotive Mobilized since the 1st Cavalry Division - and some other Cavalry units - maintain Horse Platoons. Yes, they exist for ceremonial reasons, but they are still part of their parent unit's structure. The British did not achieve 100% - and still haven't - for the same reason: Ceremonial horse units.  