To penetrate the German defenses and make a limited exploitation to the town of Coutances, General Bradley on July 13 drew an outline plan called Cobra. This plan projected a heavy attack on a narrow front just west of St.-Lo, the ground effort to be propelled forward by a mighty air attack. Bradley concentrated 6 divisions under Collins’ 7th Corps and called for support by heavy bombers. Some planes in Operation Cobra were already under way when overcast skies forced a day’s postponement. Failing to receive word of the delay, approximately 350 bombers already over the target dropped around 700 tons of bombs, some of which struck American troops. On July 25, the operation officially got under way as 2,500 planes dropped approximately 4,000 tons of bombs on a rectangular “carpet” 7 miles long and 2 miles wide along the Periers-St.-Lo highway. Though some bombs again fell short and caused casualties among the American ground troops, 3 infantry divisions followed the bombardment closely and attempted to open a hole for exploiting forces. The Germans, though badly hurt, appeared to be holding, but commitment of 2 additional American divisions on the second day and a third on the next opened a tremendous breach. General Bradley had achieved his breakthrough. Modifying his plans, he broadened the scope of the operation, and all four corps of his First Army drove ahead. By the end of the month the 7th and 8th Corps in less than a week had advanced about 30 miles. Far beyond Coutances, Americans took Avranches and gained the base of the Cotentin. This made possible not only a swing to the west into Brittany but a swing to the east, around the German left flank, toward the Seine River and Paris.
The outstanding achievement of the last week in July was the result of many factors. The Americans had outmaneuvered the Germans. Hard fighting by the 19th Corps at Tessy-surVire had blocked Kluge from sending two panzer divisions into the Cobra area to disrupt the breakthrough operation. Aggressive armored action, supported by tactical aircraft giving excellent close support, trapped considerable German forces near Coutances. Bradley’s forces had, in effect, crushed the German left flank and thereby invalidated Hitler’s tactic of standing fast until new developments in weapons might alter the situation. On August 1, Bradley turned over the command of the First Army to Lt. Gen. (later Gen.) Courtney H. Hodges. On the same day, General Patton’s Third Army became operational. Both armies went under the command of Bradley, who became the commander of the Twelfth Army Group.