Following the outbreak of the First World War, the regiment mobilised on 4 August 1914 and joined the rest of the 1st South Midland Mounted Brigade at Warwick eight days later. By the end of the month, the brigade had assembled on the Berkshire Downs with the rest of the 2nd Mounted Division, which was subsequently stationed on home defence duties on the east coast of England. As a territorial regiment, the Royal Gloucestershire Hussars was not obliged to serve overseas but could volunteer to do so, and when it did, a second-line regiment was formed in September 1914 to take its place on home defence duties. A third regiment was raised in 1915 as a reserve and to provide replacements for the first two. The regiments were numbered the 1/1st, 2/1st and 3/1st Royal Gloucestershire Hussars.