It is unknown exactly why the practice of wearing no undergarments under the kilt began, but many sources indicate it originated with the Scottish military uniform in the 18th century, leading to the invention of such expressions as "go regimental" or "military practice" for wearing no underwear. The earliest known reference to the practice is a series of satirical French illustrations in 1815 when Paris was occupied after the Battle of Waterloo. Even so, it is unknown if the practice was a dress code requirement, was left to individual soldier's preference, or merely a false rumor that later evolved into an actual practice. The kilt continued to be part of some regimental combat uniforms on the Western Front during the First World War: Every day the regiment would be inspected by a senior officer who would have a mirror to look under kilts. Anyone found wearing underpants would be sent back to take them off. However, in 1940 the kilt was retired from combat because of the vulnerability of bare skin to chemical agents, although it was retained as the formal dress uniform of the regiments. This practice led to an incident in 1997, when windy conditions during a military ceremony in Hong Kong caused a Black Watch soldier to be exposed in front of the press.