International law for armed conflicts at sea

The San Remo Manual on International Law Applicable to Armed Conflicts at Sea says in paragraph 67:

67. Merchant vessels flying the flag of neutral States may not be attacked unless they:

(a) are believed on reasonable grounds to be carrying contraband or breaching a blockade, and after prior warning they intentionally and clearly refuse to stop, or intentionally and clearly resist visit, search or capture.

There is abundant evidence from both sides that there was prior warning, that the ships refused to stop, and that they resisted the visit and being searched. And the ships were very publicly breaching a blockade.

(h/t AB)