But although submarines could impose a blockade, they could not operate according to principles of Prize Law laid down for the age of sail; forcing a vessel suspected of carrying contraband to heave to, searching her, placing a prize crew on board to take her to the nearest port where a Prize Court could adjudicate on the propriety of impounding her cargo. At best they could search their victim and give its crew time to take to the boats before sinking her. But for a submarine to surface at all was to render itself vulnerable to the smallest armed merchantman. The temptation to sink at sight and ask questions afterwards was overwhelming. The Germans restrained their submarine commanders from considerations of prudence rather than humanity: such isolated instances as the sinking of the Lusitania in 1915, which was probably carrying contraband but was certainly carrying a number of American citizens, made it clear that unrestricted submarine warfare would add the United States to the already large number of Germany’s enemies.