Cruiser rules

Cruiser rules is a colloquial phrase referring to the conventions regarding the attacking of a merchant ship by an armed vessel. Here cruiser is meant in its original meaning of a ship sent on an independent mission such as commerce raiding. A cruiser in modern naval terminology refers to a type of ship rather than its mission. Cruiser rules govern when it is permissible to open fire on an unarmed ship and the treatment of the crews of captured vessels. During both world wars, the question was raised of whether or not submarines were subject to cruiser rules. Initially, submarines attempted to obey them, but abandoned them as the war progressed.