Complex sentence: The French First Army had most of its units pocketed around Lille; those of its units evacuated from Dunkirk were relanded in France but saw no further action; they were still being reorganised in Brittany at the fall of France.
Split into simpler sentences: The French First Army had most of its units pocketed around Lille. Those of its units evacuated from Dunkirk were relanded in France but saw no further action; they were still being reorganised in Brittany at the fall of France.