"UB-5" soon joined the other UB I boats then comprising the Flanders Flotilla (), which had been organized on 29 March. When "UB-5" joined the flotilla, Germany was in the midst of its first submarine offensive, begun in February. During this campaign, enemy vessels in the German-defined war zone (), which encompassed all waters around the United Kingdom (including the English Channel), were to be sunk. Vessels of neutral countries were not to be attacked unless they definitively could be identified as enemy vessels operating under a false flag.