The French army mentality was possibly gear for the defense of France against Germany, a siege or trench army mentality ....
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Half true. The driving force behind the doctrine of the French army was fiscal. The politicians reduced allocations below that necessary to train a offensive force. One of the key selling points behind the fortification construction was cost. It was supposed to be cheaper than a standing field force of similar defense capability.
While the nazis used creative accounting & severe deficit spending to build a well trained army the French government made multiple reductions in military spending in the interest of fiscal prudence, tax control, and avoidance of government debt. The military leaders focused their shrinking resources on defense. They recognized two points: 1. The French government would not require quick offensive operations. 2. Once war came there would be money for all the training they needed.
As it was the French army had a offensive doctrine, but it was tabled until the resources to prepare it were at hand. When the war came in Sept 1939 the generals told the government it would require some 24 months to make the preparations, including training. That is the French army would be ready lead the the Allies in offensive operations in the late summer of 1941 & able to invade Germany in early 1942.