Complex sentence: Nine days after the invasion of mainland Italy, on 18 September 1943, the 3rd landed at Salerno and in intensive action drove to and across the Volturno River in October of 1943 and then to Cassino, where the battle of Monte Cassino would later be fought.
Split into simpler sentences: Nine days after the invasion of mainland Italy, on 18 September 1943, the 3rd landed at Salerno. Seeing intensive action along the way, the division drove to and across the Volturno River by October of 1943, and then to Cassino, where the battle of Monte Cassino would later be fought.