In the European Theatre, the first city to suffer heavily from aerial bombardment was Warsaw, on 25 September 1939. Continuing this trend in warfare, the Rotterdam Blitz was an aerial bombardment of Rotterdam by 90 bombers of the German Air Force on 14 May 1940, during the German invasion of the Netherlands. The objective was to support the German assault on the city, break Dutch resistance, and force the Dutch to surrender. Despite a ceasefire, the bombing destroyed almost the entire historic city centre, killing nearly nine hundred civilians and leaving 30,000 people homeless. The destructive success of the bombing led the "Oberkommando der Luftwaffe" (OKL) to threaten to destroy the city of Utrecht if the Dutch Government did not surrender. The Dutch capitulated early the next morning.