Meanwhile, Army Group A had cut the BEF's lines of communication into France and driven it back towards the coast at Dunkirk, cutting off the British troops at Calais and Boulogne from the main force. When planning the evacuation from Dunkirk (Operation Dynamo), the BEF's commanders decided that Calais and Boulogne should still be held as supply points for further fighting or possible exit points for a final withdrawal. On 17 May, 1st S/L Bty was ordered back to Calais, a move made difficult because all the roads were choked with refugees. There it operated in the S/L role to defend the port.