- British troops on the Western Front gain territory near Fontaine-lez-Croisilles, as Germany gains 200 metres north-east of Cerny
- Italian soldiers on the Southern Front repulse an Austrian attack on Carso
- A RNAS seaplane commanded by Australian Flight Sub-Lieutenant Charles Reginald Morrish sinks German submarine U-36 in the North Sea, marking the first sinking of a submarine by an aircraft
20 May 1917
