All participants were supposed to abide by the Hague Conventions of 1899 and 1907, but this was found to be impracticable for submarines. The German government maintained the British naval blockade was illegal under international law. Initially, German submarines did attempt to comply with the prize rules, but later switched to unrestricted submarine warfare following the British introduction of Q-ships with concealed deck guns. American diplomatic pressure forced the Germans to stop this for a while, but in January 1917 Germany declared a war zone around the British Isles and sank up to a quarter of shipping entering it, until escorted convoys were introduced.