The Admirals were cancelled in February 1919, before the first Lexington-class hull was laid down in August 1920. Keeping both classes therefore requires an earlier break from the historical path. Parity between the American Lexingtons and British Admirals adds further pressure, especially because the treaty's largest carriers have to be conversions. The ships cannot simply be added to the historical settlement while every other term remains unchanged.
Chapter · 5:40-8:58
The Admirals and Lexingtons break the treaty premise
The counterfactual starts by showing why keeping all Admirals and Lexingtons forces a different Washington Naval Treaty settlement.