Complex sentence: France also constructed a powerful defensive wall--a network of fortresses--along its German border called the Maginot Line, which it trusted as a way to compensates for the heavy manpower losses of the First World War.
Split into simpler sentences: France also constructed a powerful defensive wall in the form of a network of fortresses along its German border. It was called the Maginot Line and was trusted to compensate for the heavy manpower losses of the First World War.