When the German invasion plan failed. They began their night-time "Blitz" on London and other British cities including Coventry, Glasgow, Plymouth, Bristol and Birmingham. The Germans also increased U-boat attacks on Britain's vital supply routes from America of food, fuel and military equipment, causing terrible losses to merchant shipping and threatening to cut off the British Isles. Churchill was so worried by these losses that in March 1941 he ordered Bomber Command to concentrate on attacking German submarine bases and factories. Together with Coastal Command the bombers also continued to attack German shipping. The battle-cruisers Scharnhorst and Gneisenau were bombed in Brest and were unable to join the Bismarck, severely hampering the German Navy's attacks on British shipping. Low-flying bombers were also used to lay mines at sea and this proved increasingly successful at sinking German ships as the war went on, though it was a hazardous operation for the bomber aircrews. Light bombers such as Blenheims regularly launched extremely dangerous low-level daylight missions against German shipping. However the British bombers were shot out of the sky by faster German fighters. Heavy losses continued when obsolete Bomber Command aircraft, such as the single engine Battles, made desperate daylight attacks against the German forces in the Low Countries to support the British army and defend the British retreat to Dunkirk. Since the British army's retreat from Dunkirk in May/June 1940, Bomber Command had become Britain's most powerful offensive weapon. From the summer of 1940, the RAF was launching raids on Germany at night, though the bomber force was still relatively small and under-equipped. The primary targets were oil facilities and communications (e.g. railways) though as a secondary target Bomber Command was also told to attack German cities "for their intrinsic industrial and psychological value" on nights when conditions prevented identification of the primary target.