Complex sentence: Seeking an alliance with Macedonia to defend against the Aetolians, the queen mother and regent Olympias II of Epirus offered her daughter Phthia of Macedon to DemetriusII in marriage, which he accepted, but she damaged relations with the Seleucids by divorcing Stratonice of Macedon.
Split into simpler sentences: Seeking an alliance with Macedonia to defend against the Aetolians, the queen mother and regent of Epirus, Olympias II, offered her daughter Phthia of Macedon to DemetriusII in marriage. Demetrius II accepted her proposal, but he damaged relations with the Seleucids by divorcing Stratonice of Macedon.