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Agreement supplementary to The Nyon Agreement. Geneva, 17 September 1937.
III. Any surface war vessel, engaged in the protection of merchant shipping in conformity with the Nyon Arrangement, which witnesses an attack of the kind referred to in the preceding paragraph shall:
(a) If the attack is committed by an aircraft, open fire on the aircraft;
(b) If the attack is committed by a surface vessel, intervene to resist it within the limits of its powers, summoning assistance if such is available and necessary.
In territorial waters each of the participating Powers concerned will give instructions as to the action to be taken by its own war vessels in the spirit of the present Agreement.
Done at Geneva, this seventeenth day of September 1937, in the English and French languages, both texts being equally authentic, in a single copy which will be deposited in the archives of the Secretariat of the League of Nations.