The Six-Six fleet relies on coalition deterrence

Japan's Six-Six fleet programme assumes it will face one opponent at a time. Article 3 prevents an adversary from pooling its risk with allies without also bringing the Royal Navy into the war. A single enemy must therefore commit enough of its own fleet to overcome Japan. British naval strength would also threaten the European fleets and supply routes of any coalition that intervened. This protection is framed as defensive: the alliance does not promise British support for Japanese aggression.