After a slow start, the offensive gathered momentum and German resistance collapsed as scattered remnants of broken units fought to escape to the Seine.

Lacking the resources to cope with the situation, the German response was ineffectual mainly due to the effect of Operation Bluecoat and the entire Normandy front soon collapsed.

Having been delayed several times by poor weather, Operation Cobra commenced on 25 July 1944, with a concentrated aerial bombardment from thousands of Allied aircraft.

Reinforcements were moved west by Field Marshal Günther von Kluge and employed in various counterattacks, the largest of which, Unternehmen Lüttich (Operation Liège), was launched on 7 August between Mortain and Avranches.

Operation Cobra transformed the high-intensity infantry combat of Normandy into rapid maneuver warfare and led to the creation of the Falaise pocket and the loss of the German strategic position in northwestern France.

[17] Possession of Caen and its surroundings—desirable for open terrain that would permit maneuver warfare—would also give the Second Army a suitable staging area for a push south to capture Falaise, which could be used as the pivot for a swing east to advance on Argentan and then the Touques River.

[20][nb 2] The initial attempt by I Corps to reach the city on D-Day was blocked by elements of the 21st Panzer Division and with the Germans committing most of the reinforcements sent to meet the invasion to the defense of Caen, the Anglo-Canadian front rapidly congealed short of the Second Army's objectives.

[20] In part, operations were slow due to the constraints of the bocage landscape of densely packed banked hedgerows, sunken lanes and small woods, for which U.S. units had not trained.

[26] On 19 June, a severe storm descended on the English Channel, lasting for three days and causing significant delays to the Allied build-up and the cancellation of some operations.

[30] Organized German resistance finally ended on 27 June, when the U.S. 9th Infantry Division managed to reduce the defenses of Cap de la Hague, north-west of the port.

[33][34] Planning was immensely aided by detailed Ultra intelligence which supplied up-to-date decodes of communications between Oberkommando der Wehrmacht (OKW, the German armed forces high command) and Hitler's generals.

[40][41][42][43][44][45][28][excessive citations][nb 4] Montgomery approved the plan and that the strategy would remain the diversion of German attention from the First Army to the British and Canadian sector.

"[49] If VII Corps succeeded, the western German position would become untenable, permitting a relatively easy advance to the southwest end of the bocage to cut off and seize the Brittany peninsula.

The offensive began with the largest air bombardment in support of ground forces yet, with more than 1,000 aircraft dropping 6,000 short tons (5,400 t) of high explosive and fragmentation bombs from low altitude.

[24] Simultaneously, the II Canadian Corps on the western flank of Goodwood began Operation Atlantic to strengthen the Allied foothold along the banks of the Orne river and take Verrières Ridge to the south of Caen.

[76] To gain good terrain for Operation Cobra, Bradley and Collins conceived a plan to push forward to the Saint-Lô–Periers road, along which VII and VIII Corps were securing jumping-off positions.

[33] On 18 July, at a cost of 5,000 casualties, the U.S. 29th and 35th Infantry Divisions managed to gain the vital heights of Saint-Lô, driving back General der Fallschirmtruppen Eugen Meindl's II Parachute Corps.

[33] Due to poor weather conditions that had also been hampering Goodwood and Atlantic, Bradley decided to postpone Cobra for a few days—a decision that worried Montgomery, as the British and Canadian operations had been launched to support a breakout attempt that was failing to materialize.

[77] After the one-day postponement, Cobra got underway at 09:38 on 25 July, when around 600 Allied fighter-bombers attacked strongpoints and enemy artillery along a 270 m (300 yd)-wide strip of ground located in the St. Lô area.

[55] However, once again not all the casualties were German; Bradley had specifically requested that the bombers approach the target from the east, out of the sun and parallel to the Saint-Lô–Periers road, in order to minimize the risk of friendly losses, but most of the airmen instead came in from the north, perpendicular to the front line.

Additionally, a parallel approach would not in any event have assured that all bombs would fall behind German lines because of deflection errors or obscured aim points due to dust and smoke.

[82] The dead included Bradley's friend and fellow West Pointer Lieutenant General Lesley McNair—the highest-ranking U.S. soldier to be killed in action in the European Theater of Operations.

[84] Several U.S. units found themselves entangled in fights against strongpoints held by a handful of German tanks, supporting infantry and 88 mm (3.46 in) guns[84]—VII Corps gained only 2,000 m (2,200 yd) during the rest of the day.

[83] However, if the first day's results had been disappointing, General Collins found cause for encouragement; although the Germans were fiercely holding their positions, these did not seem to form a continuous line and were susceptible to being outflanked or bypassed.

[84] Even with prior warning of the U.S. offensive, the British and Canadian actions around Caen had convinced the Germans that the real threat lay there, and tied down their available forces to such an extent that a succession of meticulously prepared defensive positions in depth, as encountered during Goodwood and Atlantic, were not created to meet Cobra.

[86] Despite clear paths of advance through the floods and swamps across their front, both divisions initially disappointed the First Army by failing to gain significant ground[86] but first light the next morning revealed that the Germans had been compelled to retreat by their crumbling left flank, leaving only immense minefields to delay VIII Corps.

[92] An exhausted and demoralized Bayerlein reported that his Panzer Lehr Division, which had been ground down in the fighting against the British in June and before moving to the American front and suffering further losses around St Lo at start of July, was "finally annihilated", with its armor wiped out, its personnel either casualties or missing and all headquarters records lost.

[102] Rather than order his remaining forces to withdraw to the Seine, Adolf Hitler sent a directive to von Kluge demanding "an immediate counterattack between Mortain and Avranches" (Unternehmen Lüttich) to "annihilate" the enemy and make contact with the west coast of the Cotentin peninsula.

[109] With von Kluge's few remaining battleworthy formations destroyed by the First Army, the Allied commanders realized that the entire German position in Normandy was collapsing.

[112] By 22 August, the Falaise Pocket—which the Germans had been fighting desperately to keep open to allow their trapped forces to escape—was finally sealed, ending the Battle of Normandy with a major Allied victory.