Complex sentence: Cleomenes eventually invaded Achaea, seizing control of Corinth and Argos, but was forced to retreat to Laconia when Antigonus arrived in the Peloponnese.
Split into simpler sentences: Cleomenes eventually invaded Achaea, seizing control of Corinth and Argos. But Cleomenes was forced to retreat to Laconia when Antigonus arrived in the Peloponnese.