It went on the offensive towards Chiers on 21 August as part of the Battle of the Ardennes and the following day fought its way towards Ugny and Doncourt-lès-Longuyon, followed by fighting on the Crusnes and near the farm at Puiseaux and near Rèvemont on 23rd and near Arrancy on the 24th.

From 27 August it defended the river crossings around Gercourt and Brieulles-sur-Meuse and from 2 September resumed its withdrawal, this time south towards Montfaucon, Jubécourt and finally Rembercourt-aux-Pots.

On the declaration of war on 3 September 1939 the division at Thionville in northern France and was placed on the border opposite the German troops advancing towards Kœnigsmacker near the end of the Maginot Line.

On the morning of 12 May the first elements of the division arrived and its units deployed immediately into their positions, despite continued air attack in the Dyle region, particularly at Rhisnes and Temploux.

To the east the Germans had crossed the Albert Canal, refugees were flooding west, several cut-off Belgian units were falling back and 2nd French Cavalry Corps (made up of 2nd and 3rd Light Mechanised Divisions) was preparing to fall back after fighting all day at Hannut and Merdorp against German tanks and Stukas - 3e GRDI took over the liaison between these two light mechanised divisions.

The French cavalry corps under general Prioux also withdrew, using its last tanks to protect 12th Division's engineers, enabling them to blow the demolition charges between Hanret and Saint-Germain and regroup at Onoz.

To avoid air attack it abandoned its positions at night and the division fortified a line along the Charleroi canal running through Spy, Velaine and Fleurus.

At the end of the night 3e GRDI covered the withdrawal of 5e DINA before destroying eleven bridges between Floreffe and Ham-sur-Sambre and rendering one more impassible to road traffic.

At dawn on 16 May the division gathered at Orneau, where it received orders to rush to the area between Godarville and Motte-Courcelles on the canal to meet a German push across the Sambre to the south.

A detachment made up of elements of 106th Line Infantry Regiment and 3e GRDI under colonel Parent fought rearguard actions against advanced German units before retiring under cover of night.

The enemy penetrated the front in force across the Luttre bridge, which Belgian engineer units had failed to destroy, attacking and pressing 150th Regiment and 8th Zouaves, who formed the vanguard.

38e Combat Tanks Battalion and a group of reconnaissance squadrons from 3e GRDI under captain de Lannoy were given artillery support and managed to push the enemy back to the east bank of the canal.

The divisional commander had lost contact with his superior René Altmayer and decided to withdraw again towards Valenciennes by a night march, with 153 DIM taking up the rearguard position.