Eastern Atlantic: HM submarine E54 sinks U-81 off Western Ireland.
Mediterranean: In May 28 U-boats in Pola-Cattaro Flotilla. Allies have 858 patrol vessels (89 destroyers) of which 387 available to protect shipping, but only 201 to protect c.3,000 ships per day at sea at anyone time. During May British Admiralty orders 1108 new ASW vessels including 97 destroyers and 60 submarines. Average of 47 U-boats at sea per day.
North Sea: German seaplane sinks British SS Gena off Suffolk, but latter shoots down escorting seaplane.
Channel: In May 4 Dover Barrage drifters damaged by its mines. During May record of 13 German mine sweepers mined and sunk (only 12 so lost November 1916-April 1917).
Mediterranean: In May the transport U-boat UC-20 takes 7 Germans to set up radio station (until August) at
Misurata (Tripolitania) with Senussi rebels.
France: Early May Nivelle insists 1,000 Renault FT-17 light tanks top priority order.
Germany: In May Germans give each infantry coy 2 Bergmann sub-machine guns (another 2 in September).
Champagne: French repulse two counter-attacks south of Moronvilliers and raids on May 2.
Artois: Since April 9 BEF has advanced 2-5 miles on 20-mile front, fired 6,466,239 shells, engaged 32 German divisions (16 forced into reserve), taken 18,128 PoWs and 230 guns; 227 mortars; 470 MGs for 83,970 casualties.
Western Front: From May until July ‘B’ or Black Flight of Royal Navy Air Service No 10 squadron destroys 87 German aircraft. Royal Flying Corps flies 39,500 hours in May (record till March 1918).
North Sea: In May Germans have 47 seaplanes at Zeebrugge and Ostend, shoot down 6 French flying boats; RNAS send extra 9 seaplanes.
Macedonia: In early May after ineffective and increasingly more costly, bomber operations KG 1 is withdrawn from Hudova and railed to Western Front (RFC belatedly discovers it on May 10).
France – The ‘Ribot Cable’: French Prime minister asks US to send 4,500 combat aircraft to Western Front during 1918; with trainers, grand total required by June 30, 1918 is 22,625 planes.