On September 1st, 1939, two French Army Groups invaded the Eisenpakt. The Premiere Group d'Armées, which contained the dreaded Armée Mécanisée (consisting of 12 combined arms divisions) was tasked with eliminating German defences on the border between Alsace and France. It would be no easy feat, as the whole border between the Eisenpakt and France featured the Westwall, a massive array of WW1-style fortifications which were reactivated a few years prior. French success depended on a number of factors:
- The presence of KPD partisans and ethnically French saboteurs to paralyse German logistics across Alsace.
- Secret pre-war construction of explosive mines below strongpoints in the Westwall, eliminating many of them in one fell swoop.
- Preparation of combat engineer units trained specifically to bridge and demolish minor obstacles such as ditches and tank traps.
- An exorbitant number of tactical airstrikes and artillery bombardment to suppress the defenders.
- Employment of tanks to breakthrough fortified sections of the line, using direct fire to reduce bunkers and pillboxes to rubble.
- Deployment of paratroopers to capture key facilities such as airbases, train stations and undermanned fortifications.
The German Abwehr was unable to comprehend the true scale and complexity of French preparations and was alerted only days prior of many of these, though even then a number of factors came as a complete surprise (particularly the use of explosive mines built pre-war under the Westwall). As a final nail in the coffin, large amounts of Chlorine Trifluoride (ClF3) produced in the Soviet Union and secretly shipped to France were used to test the compound's effectiveness against fortifications; the results on the ill-prepared German defenders were brutal, as if the compound didn't immediately immolate their positions, toxic gases and acidic corrosion got them.
With their defences undermanned, pummelled by enormous firepower, breached at tremendous speeds by combined arms formations and harassed on all directions by irregular forces, German forces retreated after less than two weeks of fighting. Reinforcements were slow to arrive and were endlessly harassed from the air thanks to a French aerial interdiction campaign which targeted trains, train stations and bridges across the Rheinland. Germany's quick defeat at the Battle of Alsace would be the first of many such embarrassments.