The British attempted to motorize its entire force but the Depression stalled that attempt The USA  indeed  achieved   Automotive Mobility between  and  and the only reason they did not achieve  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  Automotive Mobilized since the st Cavalry Division  and some other Cavalry units  maintain Horse Platoons Yes they exist for ceremonial reasons but they are still part of their parent units structure The British did not achieve   and still havent  for the same reason Ceremonial horse units