Answer
Antigonus was Seleucus' main rival among the Diadochi, Alexander the Great's generals who fought for control of his empire. Antigonus sought to reunite Alexander's empire under his rule, while Seleucus aimed to establish his own kingdom in the east. Their rivalry culminated in the Battle of Ipsus in 301 BC, where Seleucus and his allies defeated Antigonus, solidifying Seleucus' power and establishing the Seleucid Empire.