Article 21 and a naval build-up outside the treaty

Article 21 provided an escape mechanism for a threat created by naval construction outside the agreement. A signatory that judged its security materially affected could notify the others and seek higher tonnage, with the other parties able to respond proportionally. France, Italy, and Germany's Deutschland-class construction appear as relevant examples, but Soviet fleet ambitions were the sharper immediate concern for Japan. The clause also helped Japanese negotiators argue at home that a future Soviet Pacific build-up would permit a corresponding increase. Restraint among signatories came with a warning that an outsider's expansion could trigger collective rearmament.