The Formidable class of battleships were a three-ship class of pre-dreadnoughts designed by Sir William White and built for the Royal Navy in the late 1890s. The class comprised Formidable, Irresistible, and Implacable. They were armed with a battery of four 12-inch (305 mm) guns, they had top speed of 18 knots, and they marked the adoption of Krupp armour in British battleship designs. The class formed the basis for the nearly identical London class of five ships, and those ships are sometimes included in the Formidable class. Formidable, Irresistible, and Implacable were built between 1898 and 1901 at the Portsmouth, Chatham, and Devonport Dockyards, respectively.

Mikasa (三笠) is a pre-dreadnought battleship built for the Imperial Japanese Navy (IJN) in the late 1890s, and is the only ship of her class. Named after Mount Mikasa in Nara, Japan, the ship served as the flagship of Vice Admiral Tōgō Heihachirō throughout the Russo-Japanese War of 1904–1905, including the Battle of Port Arthur on the second day of the war and the Battles of the Yellow Sea and Tsushima. Days after the end of the war, Mikasa's magazine accidentally exploded and sank the ship. She was salvaged and her repairs took over two years to complete. Afterwards, the ship served as a coast-defence ship during World War I and supported Japanese forces during the Siberian Intervention in the Russian Civil War.