 Complete the following phrase:  The BEF participated in the Dyle Plan, a rapid advance into Belgium to the line of the Dyle River, but the 1st Army Group had to retreat rapidly through Belgium and north-western France, after the German breakthrough further south at the Battle of Sedan 1215 May. A local counter-attack at the Battle of Arras 1940 21 May was a considerable tactical success but the BEF, French and Belgian forces north of the Somme River retreated to Dunkirk on the French North Sea coast soon after, British and French troops being evacuated in 