https://en.wikipedia.org/wiki/French_war_planning_1920%E2%80%931940

Rowe, V. (1959). The Great Wall of France: The Triumph of the Maginot Line (1st ed.). London: Putnam. OCLC 773604722.

Ignoramus; the French calculated that a man in a fortified position was worth seven in the open so by building the line they needed only 1/7th of the men to garrison the French-German frontier. The rest of the army was armoured and motorised to fight in Belgium to protect the Lille industrial conurbation and to foist the destruction of the war onto another country. The demographic hole that was caused by French casualties in the Great War was at its worst in the early 40s and despite losing 20% of its land Germany still had a population of about 65 million in 1940 against about 43 million French. The Maginot Line was a clever way to multiply the limited French manpower to avoid the worst consequences of a smaller population.