Germany: Government informs US Government it will not ‘leave any means whatever unturned [ie U-boat terror] to hasten the end of the war. Since the Allies have rebuffed Germany’s attempt to reach an understanding by negotiation … The Imperial German Government must therefore abandon the limitations … imposed upon itself …’.
Germany announces zone in which all shipping liable to be sunk as from Terschelling (Holland) north to Udsive (Norway) then northwest to Faroe Islands and down longitude 20° West 350 miles west of Ireland to Cape Finisterre. Zone also includes whole Mediterranean excluding Balearic Islands and seas east of Gibraltar as far as Spain’s heel. A 20-mile channel left for ships sailing to Greece until January 11, 1918. Archangel added to barred zone on March 1917. Scheer Order of Day says whole Navy must support U-boats.
During January U-boats have sunk 180 ships (51 British, 63 Allied, 66 neutral including 34 Norwegian) worth 328,391t (British 109,954t), 2 U-boats lost. U-boat toll in Mediterranean is 75,541t.
GERMANY DECLARES UNRESTRICTED U-BOAT WAR and threatens cross-Channel hospital ships (alleged munition cargoes). Bernstorff orders sabotage of German ships in US ports.
Britain: Commodore Godfrey Paine Fifth Sea Lord and Director of Air Services (succeeding Rear-Admiral Vaughan-Lee).
Western Front: During January 4 German Jagdstaffeln (fighter squadrons) formed, another 6 in February.