 The German breakthrough at Sedan , which had been thought impassable , meant that defenders of the K @-@ W Line risked being outflanked , and had to withdraw on 16 May . The German invasion triggered a panic amongst Belgian civilians in the path of the advancing German army . By 11 May , the roads leading westwards , away from the fighting , were blocked by refugees , hampering the eastward advance of French and British forces . It is estimated that around two million civilians fled their homes during the campaign . 
