Article: Philip II was a strong and expansionist king and he took every opportunity to expand Macedonian territory. In 352 BC he annexed Thessaly and Magnesia. In 338 BC, Philip defeated a combined Theban and Athenian army at the Battle of Chaeronea after a decade of desultory conflict. In the aftermath, Philip formed the League of Corinth, effectively bringing the majority of Greece under his direct sway. He was elected Hegemon of the league, and a campaign against the Achaemenid Empire of Persia was planned. However, while this campaign was in its early stages, he was assassinated.

Now answer this question: Who was Philip II planning a campaign against when he was assasinated?
Achaemenid Empire of Persia