September 1939, funnily enough:
"World War II began last week at 5:20 a. m. (Polish time) Friday, September 1, when a German bombing plane...."
Some in Europe already saw it as an incipient global conflict after Italian and Japanese ventures in Ethiopia and China: Duff Cooper (Who'd resigned as First Lord of the Admiralty after the Munich agreement in 1938) titled his 1939 published collection of articles & speeches "The Second World War: First phase" - and that ended in August 1939.