 Beginning on 30 July , the British Second Army , under Lieutenant @-@ General Miles C. Dempsey , mounted a supporting attack ( code @-@ named Operation Bluecoat ) on the eastern flank of the American armies . Much of the German armored reserves being rushed west to halt the American breakthrough were diverted to face this new threat . Meanwhile , the U.S. continued its attacks to widen the corridor around Avranches . Although the Germans held the vital road junction of Vire , U.S. VII Corps , under Major General J. Lawton Collins , captured Mortain , 19 miles ( 31 km ) east of Avranches , on 3 August . 
