qnli question: What caused early aircraft carriers to be made up of conversions of ships that were previously cargo ships, cruisers or battlecruisers? sentence: 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.