Complex sentence: The French army had not completed an adaptation to siege warfare and much of the equipment necessary, particularly heavy artillery, did not exist, which made it impossible to synchronise operations in Artois with the First Battle of Champagne, which ended on 17 March.
Split into simpler sentences: The French army had not completed an adaptation to siege warfare and much of the equipment necessary, particularly heavy artillery, did not exist. It had been impossible to synchronise operations in Artois with the First Battle of Champagne, which ended on 17 March.