Complex sentence: By 600 BC the Loire had already become a very important trading route between the Celts and the Greeks and would become one of the great highways of France for over 2000 years.
Split into simpler sentences: By 600 BC the Loire had already become a very important trading route between the Celts and the Greeks. A key transportation route, it served as one of the great "highways" of France for over 2000 years.