On 1 August, VIII Corps became part of Lieutenant General George S. Patton's Third Army. On 4 August, Montgomery changed the invasion plan by sending a corps to occupy Brittany and push the German troops around the ports, while the rest of the Third Army continued east. Because of the large number of German forces south of Caen, Montgomery moved the British armor west and launched Operation Bluecoat from 30 July until 7 August to add to the attacks from the United States armies. This pushed the German forces to the west, allowing the launch of Operation Totalize south from Caen on 7 August.
By the beginning of August, more German reserves became available. The German forces were being encircled and the German High Command wanted these reserves to help with a retreat to the Seine. Hitler demanded an attack at Mortain on 7 August. The attack was pushed back by the Allies, who again had advance warning from Ultra code breaking. The original Allied plan was for encircling the Germans as far as the Loire valley. Bradley realized that many of the German forces in Normandy were not able to move by this stage and he got Montgomery's approval by telephone on 8 August for to encircle German forces. This was left to Patton to do. He moved nearly unopposed through Normandy. The Germans were left in near Chambois. Strong German defense and the sending of some American troops for an attack by Patton towards the Seine at Mantes prevented the Germans from being trapped until 21 August. On this date, 50,000 German troops were trapped.
Whether this could have been done earlier with more prisoners taken has been debated.
The liberation of Paris followed shortly afterwards. The French Resistance in Paris rose attacked the Germans on 19 August. The French 2nd Armoured Division under General Philippe Leclerc, along with the U.S. 4th Infantry Division accepted the surrender of the German forces there and liberated Paris on 25 August.
Operations continued in the British and Canadian sector until the end of the month. On 25 August, the 2nd U.S. Armored Division fought its way into Elbeuf, making contact with both British and Canadian armoured divisions there. The 2nd Canadian Infantry Division advanced into the Forêt de la Londe, on the morning of 27 August. The area was strongly held and the 4th and 6th Canadian brigades had large numbers of dead and wounded over three days as the Germans defended their position. The Germans pulled back on the 29th, withdrawing over the Seine on the 30th.
On the 30th the 3rd Canadian Infantry Division crossed the Seine near Elbeuf and entered Rouen to a happy welcome.
The campaign in Normandy is considered by historians to end either at midnight on 24 July 1944 (the start of "Operation Cobra" on the American front), 25 August 1944 (the liberation of Paris), or 30 August 1944, the date the last German unit retreated across the River Seine.
The original "Overlord" plan was for a ninety-day campaign in Normandy with the ultimate goal of reaching the Seine; this goal was met early. American forces were fighting in Brittany as anticipated by General Montgomery during the latter weeks of the campaign. Historians consider the Normandy campaign to have ended with the massive breakout of "Operation Cobra".
The US official history describes the fighting beginning on 25 July as the "Northern France" campaign, and includes the fighting to close the Falaise Gap, which the British/Canadians/Poles consider to be part of the Battle of Normandy. Volume I of the Official History of the Canadian Army in the Second World War by C.P. Stacey, published in 1955, as well as the Canadian Army's official Historical Summary of the Second World War, published in 1948, define the Battle of Normandy as lasting from 6 June 1944 to 1 September 1944. The definition of the Battle of Normandy is also evident in another publication by the Army's Historical Section entitled "Canada's Battle in Normandy".
There were reports of Eisenhower requesting Montgomery's replacement in July. The lack of forward progress was caused by the rough land. However, as at the battle of El Alamein, Montgomery kept to his original attrition warfare strategy, reaching the objectives within his original ninety day target.
Victory in Normandy was followed by a pursuit to the French border in short order. Germany was forced once again to reinforce the Western Front with manpower and resources from the Soviet and Italian fronts.
By September, Allied forces of seven field armies (two of which came through southern France in Operation Dragoon) were approaching the German frontier. The Allied battle plan was good, drawing on the strengths of both Britain and the United States. German leadership was often faulty, despite good fighting by German units.
In a larger context the Normandy landings helped the Soviets on the Eastern front, who were facing most of the German forces. It shortened of the conflict there.
Victory in Normandy stemmed from several factors. The Allies had more weapons and equipment. They also had new inventions like the PLUTO pipelines and Mulberry harbours. These helped the flow of troops, equipment, fuel and ammunition. Movement of cargo over the open beaches went better than expected,even after the destruction of the U.S. Mulberry in the channel storm in mid-June.
By the end of July 1944, one million American, British, Canadian, French, and Polish troops, hundreds of thousands of vehicles, and a lot of supplies were ashore in Normandy. Artillery ammunition and other items were plentiful. This was impressive considering they did not hold a port until Cherbourg fell.
By the time of the Normany breakout the Allies also had superiority in numbers of troops (approximately 3.5:1) and armored vehicles (approximately 4:1) which helped overcome the natural advantages the rough land gave to the German defenders.
Allied intelligence and counterintelligence efforts were successful. The Operation Fortitude trick plan before the invasion kept German attention focused on the Pas-de-Calais. High-quality German forces were kept in this area, away from Normandy, until July. Prior to the invasion, few German reconnaissance flights took place over Britain, and those that did saw only the fake camps. Ultra decoding of German Enigma (machine) communications had been helpful as well, exposing German plans.
Lack of an organized strategy hurt the German defence. German leadership was split between Field Marshals von Rundstedt and Rommel. Von Rundstedt wanted to keep the powerful units in reserve, to do a powerful counterattack once the allied landing started. Rommel wanted to stop the allies at the beach. He tried to locate units so that they could attack quickly.
While Rommel's plan was good, the reserve strategy was not good because units could not move during the day due to allied air strikes. In the end, the mix of two strategies was a disaster. The beach defences were overcome and the counterattacks were not strong enough.
German commanders at all levels failed to react to the attack quickly. Communication problems added to the problems caused by Allied air and naval firepower. Local commanders did not lead an aggressive defence on the beach. The German High Command focused on the Calais area, and von Rundstedt was not permitted to use the armored reserve.
When it was finally released late in the day, success was more difficult. Although the 21st Panzer Division, had attacked earlier, it faced strong opposition that had been allowed to build at the beaches. Overall, even though the Allied material superiority continued to grow, the Germans slowed the Allies advance for nearly two months, aided by the rough land.
Although there were several disputes among the Allied commanders, their plans were decided by the main commanders. By contrast, the senior German leaders always had interference by Hitler, who did not know local conditions.