Complex sentence: In 1938, the French judged Jebel Dahar to be impassable to motorised transport and so had not extended the Mareth Line any further inland but in 1943, motor vehicles had much better performance.
Split into simpler sentences: In 1938, the French judged Jebel Dahar to be impassable to motorised transport and so had not extended the Mareth Line. But in 1943, motor vehicles had much better performance.