High Quality Content by WIKIPEDIA articles! The Hardest Day was the name of a Second World War air battle fought during the Battle of Britain on 18 August 1940, by the Luftwaffe and British Royal Air Force (RAF). By June 1940 the Allies had been defeated in Western Europe and Scandinavia. Rather than come to terms with Germany, Britain rejected all overtures for a negotiated peace resulting in Adolf Hitler issuing the German armed forces (Wehrmacht) Directive No. 16 ordering the invasion of the United Kingdom.