The Second London Treaty frames the Illustrious compromise

The Second London Naval Treaty removed cumulative carrier tonnage limits while imposing a nominal 23,000-ton limit per ship and a 6.1-inch gun ceiling. Its effectiveness depended on Japanese and Italian participation, since other navies would eventually have to respond if those countries ignored the restrictions. The exact declared displacements of Illustrious and the Lexington conversions invite skepticism about treaty accounting. Royal Navy studies considered carriers of up to 30,000 tons, then tested what capabilities could most safely be sacrificed to meet the limit. Illustrious retained protection against bomb damage and continuous sortie generation at the cost of a smaller air group. Deck-edge-lift options are less certain: Clarke had found notes suggesting their consideration but had not found the corresponding drawings.