The 'Battle of Mantes-Gassicourt' was fought between US and German forces in northern France (18/20 August 1944).

In this undertaking, Lieutenant General George S. Patton’s US 3^rd Army secured the first Allied bridgehead across the Seine river in the aftermath of 'Overlord', which allowed the Allies to engage in the 'Liberation of Paris'. During the two days of the bridge crossing, US anti-aircraft artillery shot down more than 40 German warplanes.

After the 'Neptune' (iii) successful Allied landing in Normandy on 6 June 1944, the US 3^rd Army was formed in France to help in the exploitation of the US break-out from Normandy in 'Cobra'. The drive to the Seine river began on 3 August, when Lieutenant General Omar N. Bradley, commander of the US 12^th Army Group that had been created only one day earlier, ordered Patton to secure the north/south line of the Mayenne river, to clear the area to the west of the Mayenne river as far to the south as the Loire river, and to protect the 12^th Army Group’s southern flank with minimum forces. Since Major General Troy H. Middleton’s VIII Corps was driving to the south-west toward Rennes in Brittany and Major General Wade H. Haislip’s US XV Corps was about to move to the south-east toward Mayenne, Patton oriented Major Walton H. Walker’s XX Corps to the south toward Nantes and Angers. As the main US effort veered eastward in accordance with the modified 'Overlord' plan and the XV Corps drove toward Laval and Le Mans, Patton ordered the XX Corps to cross the Mayenne river in a parallel drive to protect the XV Corps' southern flank.

The XV Corps drove from Mayenne southward to Le Mans and then northward to Argentan by 12 August 1944 where the 'Battle of the Falaise Pocket' was developing against the German 7^th Army. On the next day, Major General Lunsford E. Oliver’s US 5^th Armored Division of the XV Corps advanced 35 miles (56 km) and reached positions overlooking Argentan. On 13 August, Bradley overruled orders by Patton for a farther push to the north toward Falaise by the 5^th Armored Division. Bradley ordered the XV Corps to 'concentrate for operations in another direction'. The US forces near Argentan were ordered to withdraw, which ended the pincer movement by XV Corps. Patton objected but complied, which left a limited exit for the German forces otherwise trapped in the Falaise pocket.

While the XV Corps left part of its forces at Argentan and started the wider envelopment to the Seine river on 15 August, other components of the 3^rd Army farther to the south were also driving toward the Seine river, sweeping clear the vast area to the north of the Loire river. The advance to the Seine fulfilled a prophecy made a week earlier that 'the Battle of Normandy is rapidly developing into the Battle of Western France'.

Having decided to send part of Patton’s force down the western bank of the Seine river, the Allied commanders saw a coincident opportunity to seize a bridgehead on the river’s eastern bank as a springboard for future operations. The XV Corps was therefore allocated a double mission: the 5^th Armored Division was to attack down the river’s western bank while Major General Ira T. Wyche’s US 79^th Division was to establish a bridgehead on the river’s eastern bank. In his order issued on 20 August, General Sir Bernard Montgomery, commanding the Allied ground forces in northern France, cautioned that 'This is no time to relax, or to sit back and congratulate ourselves…Let us finish off the business in record time.' By then, US troops were already across the Seine river.

Wyche had received a telephone call at 21.35 on 19 August from Haislip, who ordered him to cross the Seine river that night. The 79^th Division was to get infantry onto the eastern bank at once, build a bridge for vehicles, tanks and heavy equipment, and gain ground in sufficient depth, 4 to 6 miles (6.4 to 9.7 km), to protect the crossing sites at Mantes from medium artillery fire.

In a situation that was 'too fluid to define a [German] front line,' Wyche anticipated little resistance. His 79^th Division had that day engaged only scattered German groups in flight, had captured 19 vehicles and a PzKpfw IV battle tank, and received only sporadic machine gun fire from across the Seine river, which was itself the main problem, for near Mantes it varied in width from 500 to 800 ft (150 to 245 m). Fortunately, a nearby dam provided a narrow footpath across the waterway, and engineer assault boats and rafts could transport troops and light equipment. For the bridge he was to build, Wyche secured some 700 ft (215 m) of treadway from the 5^th Armored Division.

While torrential rain fell during the night of 19/20 August, men of the 313^th Infantry walked across the dam in single file, each man touching the one ahead to keep from falling into the water. At daybreak on 20 August, as the 314^th Infantry paddled across the river, the division engineers began to install the treadway. In the afternoon, as soon as the bridge was ready, the 315^th Infantry crossed in trucks, and by the fall of night on 20 August the bulk of the division and its attached units, including tanks, tank destroyers and artillery, was on the eastern bank. On the following day, battalions of the XV Corps' artillery crossed.

Under the command of Colonel Joseph Bacon Fraser, the 23^rd Anti-Aircraft Artillery Group hurriedly emplaced its weapons around the bridge, arriving in time to shoot down about a dozen German aircraft on the first day and to amass a total of 43 German warplanes shot down in two days. The number of aircraft brought down in a day was a record for a US anti-aircraft artillery group in World War II.

To supplement the treadway bridge, engineers constructed a Bailey bridge which was opened to traffic on 23 August. On the eastern bank, the 79^th Division not only extended and improved the bridgehead, repelled counterattacks, and interdicted highways, ferry routes and barge traffic lanes, but also dramatically emphasised to the Germans the critical nature of their situation by capturing the command post of Generalfeldmarschall Walter Model’s Heeresgruppe 'B' at la Roche-Guyon and sending the German headquarters troops fleeing eastward to Soissons. Once the Allies were on the eastern side of the Seine river, Général de Division Philippe François Marie Jacques Leclerc de Hauteclocque’s 2ème Division Blindée and Major General Raymond O. Barton’s US 4^th Division liberated Paris on 24/25 August.