 Also on 25 July the Americans launched their break @-@ out offensive , Operation Cobra , which gained immediate success . By the end of the third day of the operation , American forces had advanced 15 mi ( 24 km ) south of the Cobra start line at several points . On 30 July , American forces captured Avranches , at the base of the Cotentin peninsula . The German left flank was now open and within 24 hours , units of the United States Third Army had entered Brittany and began advancing south and west through open country almost without opposition . Three German Panzer divisions — the 1st SS , 9th SS and 116th — were shifted westward from Verrières Ridge to face this new threat . 
