Complex sentence: Herod went back to Judea to win his kingdom from Antigonus and at the same time he married the teenage niece of Antigonus, Mariamne (known as Mariamne I), in an attempt to secure a claim to the throne and gain some Jewish favor.
Split into simpler sentences: Herod went back to Judea to win his kingdom from Antigonus. He did this in an attempt to secure his claim to the throne and gain some Jewish favor.