Somme – Battle of Cantigny: First AEF offensive operation, by US 28th Infantry Regiment (3,874 men; 823 casualties), 1st Division captures village and beats off 3 counter-attacks by German 82nd Reserve Division (c.1,000 casualties) until May 29.
Aisne: Germans now have 40-mile wide, 15-mile deep bridgehead across Aisne, also cross the Vesle. Ex-BEF GHQ liaison officer General des Vallieres, French 151st Division Commander, killed by German machine-gun at Juvigny. Allied stand at Vregny plateau and heights of St Thierry as 3 reserve divisions arrive. German officer reports ‘regrettable excesses … serious drunkenness’.
Italian Front: Diaz informs Foch and CIGS Sir H Wilson that he has postponed his own offensive (towards Valsuguna) to deal with imminent Austrian one.
ARMENIA DECLARES HER INDEPENDENCE.
Germany: 10 DH9s of No 99 Squadron attack rail targets at Bensdorf.
Western Front: Germans capture Magneux airfield on Aisne with all its aircraft. A second French bomber group joins Aisne air battle.