According to the UN laws of the sea, piracy is a war-like act committed by private parties (not affiliated with any government) that engaged in acts of robbery and/or criminal violence at sea
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Next time, .50 cal machine guns should be used on the pirates.
I'd say the eco-nuts intentional ramming of the Japanese ship qualifies as a "war-like act" as well as an act of "criminal violence at sea." Since the eco-nuts have established a history and pattern of violence at sea, I believe the Japanese should take more direct action to protect themselves - possibly using machine guns (eg. 50 cal or so) to shoot out the engines and disable any craft closing on it with obvious intent to ram.