The development of flattop vessels produced the first large fleet ships. In 1918, HMS Argus became the world's first carrier capable of launching and recovering naval aircraft. As a result of the Washington Naval Treaty of 1922, which limited the construction of new heavy surface combat ships, most early aircraft carriers were conversions of ships that were laid down (or had served) as different ship types: cargo ships, cruisers, battlecruisers, or battleships. These conversions gave rise to the Lexington-class aircraft carriers (1927), Akagi and Courageous class. Specialist carrier evolution was well underway, with several navies ordering and building warships that were purposefully designed to function as aircraft carriers by the mid-1920s, resulting in the commissioning of ships such as Hōshō (1922), HMS Hermes (1924), and Béarn (1927). During World War II, these ships would become known as fleet carriers.[citation needed]
What caused modern aircraft carriers to be made up of conversions of ships that were previously cargo ships, cruisers or battlecruisers?