Explanation: In WW2, Nazi Germany, due to a variety of factors, some of them quite strange, outflanked the French and British forces, collapsing the Franco-British front line and forcing them into retreat. During this time, French forces were so strung out over such a wide area that many ran out of gas (since even the supply units were scrambling to get to new positions) and were caught piecemeal by literally-methed-up Nazi troops advancing for days without rest.

Wherever the Nazis encountered significant concentrations of French forces, the French military was sure to make the Nazis pay dearly - the issue was getting the scattered French army from Point A to Point B to regroup with three million Nazis between them. For this reason, the decision was eventually made for the cornered and isolated British Expeditionary Force (BEF) to evacuate France by sea at Dunkirk, with the Nazis hot on their heels.

After a failed attempt to break through the Nazi lines to connect with the rest of the French military, the French First Army remained with the BEF. The previously-allied Belgians surrendered with surprising suddenness due to their monarch at the time being a literal fascist sympathizer, leaving the French and British scrambling to fill the suddenly-opened gaps in their defenses.

While some French troops were evacuated at Dunkirk, a large percentage of them simply fought, while surrounded, greatly outnumbered, and outgunned, until their food and ammunition ran out entirely, which stalled Nazi forces long enough for Allied troops to escape the Nazi vise and continue the fight against the fascists after regrouping in Britain.

“Ils ne passeront pas” is a French phrase meaning “They will not pass” - first used by the French at Verdun in WW1 against Imperial German forces, it acquired wider currency in the interwar period as an anti-fascist slogan, especially during the Spanish Civil War, in the form of “No pasarán!”