Complex sentence: Designed in preparation for World War II, and intended for use against the deep forts of the Maginot Line, they were not ready for action when the Wehrmacht outflanked the line during the Battle of France.
Split into simpler sentences: Designed in preparation for World War II, they were intended to be used against the deep forts of the Maginot Line. However, they were not ready for action when the Wehrmacht outflanked the line during the Battle of France.