The Invasion of Normandy (Operation OVERLORD). Despite unfavorable weather forecasts, General Eisenhower made the decision to attack on 6 June 1944. At 0200 that morning one British and two American airborne divisions were dropped behind the beaches in order to secure routes of egress from the beaches for the seaborne forces. After an intensive air and naval bombardment, assault waves of troops began landing at 0630. More than 5,000 ships and 4,000 ship-to-shore craft were employed in the landings. British forces on the left flank and U.S. forces on the right had comparatively easy going, but U.S. forces in the center (OMAHA Beach) met determined opposition. Nevertheless, by nightfall of the first day, large contingents of three British, one Canadian, and three American infantry divisions, plus three airborne divisions, had a firm foothold on Hitler's "fortress Europe."

During the weeks that followed the landings, the Germans fiercely resisted Allied advances in the hedgerows of Normandy. Cherbourg fell three weeks after the landings, but the port had been destroyed and time-consuming repairs were required before it could be used to relieve the Allied supply problem. Meanwhile, Allied forces had been deepening the beachhead. By the end of June the most forward positions were about 20 miles inland. The buildup of Allied forces was swift, despite the lack of ports, and by 1 July almost a million men, more than a half-million tons of supplies, and 177,000 vehicles had been landed. By this time General Bradley's U.S. First Army comprised 4 corps with 11 infantry and 2 armored divisions. British strength was about the same.

At the end of June, British forces made an attempt to break into the open country near Caen. Heavy bombers were used in close support to facilitate this breakout, but the destruction they wrought served to impede rather than to assist the British ground forces and German armored units blocked an advance in that sector. General Montgomery now adopted the strategy of attracting German armor to the British sector while American units continued to attack in the vicinity of St. Lo. On 25 July a massive air bombardment was coordinated with an attack by ground troops that achieved a distinct penetration of German lines. General Patton's U.S. Third Army poured through this breach in the direction of Brittany with the object of securing the much-needed ports in that area.

The Allied strategic plan was to take over Breton ports and then to secure a lodgment area as far east as the Seine River, to provide ample room for air and supply bases. It was then intended to advance into Germany on a broad front. The principal thrust east was to be north of the Ardennes Forest in Belgium with General Montgomery's British 21st Army Group. A subsidiary thrust by General Bradley's newly formed U.S. 12th Army Group, comprising the U.S. First and Third Armies, was to be made south of the Ardennes. This northern route was chosen because it led directly into the Ruhr area where Germany's industrial power was concentrated.

The Allied strategic plan underwent considerable modification early in August to seize upon the advantages of the breakout and exploit the principle of maneuver. When the Germans counterattacked with the intention of restoring a stable front and cutting off U.S. forces moving toward Brittany, they unwittingly offered the Allies an opportunity to encircle them. British forces on the left moved toward Falaise and U.S. troops to the right executed a wide circling maneuver toward Argentan, roughly halfway between St. Lo and Paris. Caught in a giant pocket, the Germans nevertheless extricated many troops before the Argentan-Falaise gap was closed on 20 August, though losing more than 70,000. Meanwhile, General Patton's Third Army drove eastward across the Seine and eliminated it as a German defensive line, encircling and destroying Germans who had escaped the Argentan-Falaise pocket. The Germans lost almost all of two field armies in Normandy.

Originally it had been intended to bypass Paris in order to spare the city from heavy fighting, but, with the crossing of the Seine, fighting broke out in the city between French patriots and Germans stationed there. Lest the uprising be defeated, a column of U.S. and Free French troops were deflected toward Paris, entering the city on 25 August 1944.

General Eisenhower now altered his original plan, abandoning the idea of stopping at the Seine and instituting instead a determined pursuit of the enemy toward Germany. Because the ports of Cherbourg and Brest no were too far west to support the accelerated movement, the new plans involved capture of Channel parts and especially of Antwerp, the best port in Europe. Exploiting the new situation, General Eisenhower now reinforced the British by sending the U.S. First Army close alongside the 21st Army Group toward Aachen in a drive toward Antwerp. Only the U.S. Third Army continued east on the subsidiary axis south of the Ardennes.

Cherbourg remained the only major port supplying Allied forces in northern France, and advances to the east had been so rapid that the supply services simply could not keep up. The drive eastward began to grind to a halt for lack of supplies, chiefly gasoline. The British took Le Havre and several Channel ports and on 4 September 1944 they captured Antwerp, its port intact. But Antwerp could not yet be used to relieve a growing logistical crisis because the Germans denied access to the sea by retaining control of the Schelde Estuary. The newly activated U.S. Ninth Army (Lt. Gen. William H. Simpson commending) in Brittany took Brest late in September, but the port had been completely destroyed, and in any event its location so far from the scene of action precluded its usefulness in solving logistical problems.