Complex sentence: In response, Caracalla used the opportunity to start a campaign against the Parthians and in the summer Caracalla began to attack the countryside east of the Tigris in the Parthian war of Caracalla.
Split into simpler sentences: In response, Caracalla used the opportunity to start a campaign against the Parthians. In the summer Caracalla began to attack the countryside east of the Tigris in the Parthian war of Caracalla.