[Hitman]

Yes as long as they do transport materials that are useful for the war effort (There were lists of them like steel, zinc, chemical components for explosives, etc). No if they are transporting goods not included in those lists (F.e. grain, meat, etc). Theoretically the commander should stop the merchant and ask for the papers to inspect the load. If not legitimate, the ship could be sunk and the US would not be able to legally protest for that (Prisen Ordnung).