From Japan's perspective, the alliance limits any war to one opponent unless the Royal Navy joins. Even during British neutrality, British shipping could continue trading with Japan. An enemy attempting a complete blockade would then risk interfering with neutral British commerce and widening the war. The alliance therefore provides deterrent value without requiring Britain to fire a shot. Britain's own answer to inconvenient neutral commerce is described as buying the trade itself. The partners also accept separate spheres in China and Korea, provided their interests do not collide, with little concern for Chinese or Korean consent.
Notable Point · 32:42-35:06
British neutrality as Japanese leverage
British neutral shipping could make a blockade of Japan diplomatically risky while Britain stayed outside the fighting.