The British Empire declare war on Iran following Naser al-Din Shah's capture of Herat.

The British Empire comprised the dominions, colonies, protectorates, mandates, and other territories ruled or administered by the United Kingdom and its predecessor states. It began with the overseas possessions and trading posts established by England in the late 16th and early 17th centuries, and colonisation attempts by Scotland during the 17th century. At its height in the 19th and early 20th centuries, it was the largest empire in history and, for a century, was the foremost global power. By 1913 the British Empire held sway over 412 million people, 23 per cent of the world population at the time, and by 1920 it covered 35.5 million km² (13.7 million sq mi), 24 per cent of the Earth's total land area. The British Parliament was the supreme law-making authority for all the British possessions which made up the Empire. As a result, its constitutional, legal, linguistic, and cultural legacy is widespread. It was described as "the empire on which the sun never sets", as the sun was always shining on at least one of its territories.