Complex sentence: It came ashore at Salerno on 15 September 1943, to help repel heavy German counterattacks, then as part of US Fifth Army's British X Corps and supported by the 46th (North Midland) Division, drove on and took Naples.
Split into simpler sentences: It came ashore at Salerno, on 15 September 1943, to help repel heavy German counterattacks. Then, as part of US Fifth Army's British X Corps and supported by the 46th (North Midland) Division, it drove on and took Naples.