While negotiating with Polish Ambassador Józef Lipski and advising accommodation over Germany's territorial ambitions, as he had during the "Anschluss" with Austria and the occupation of Czechoslovakia, Germany staged the Gleiwitz incident, and the invasion of Poland began on 1 September. Henderson had to deliver Britain's final ultimatum on the morning of 3 September 1939 to Ribbentrop that if hostilities between Germany and Poland did not cease by 11 a.m. that day, a state of war would exist between Britain and Germany. Germany did not respond and so Chamberlain declared war at 11:15 a.m. Henderson and his staff were briefly interned by the "Gestapo" before they returned to Britain on 7 September.