Hellenistic warfare was a continuation of the military developments of Iphicrates and Philip II of Macedon, particularly his use of the Macedonian Phalanx, a dense formation of pikemen, in conjunction with heavy companion cavalry. Armies of the Hellenistic period differed from those of the classical period in being largely made up of professional soldiers and also in their greater specialization and technical proficiency in siege warfare. Hellenistic armies were significantly larger than those of classical Greece relying increasingly on Greek mercenaries (misthophoroi; men-for-pay) and also on non-Greek soldiery such as Thracians, Galatians, Egyptians and Iranians. Some ethnic groups were known for their martial skill in a particular mode of combat and were highly sought after, including Tarantine cavalry, Cretan archers, Rhodian slingers and Thracian peltasts. This period also saw the adoption of new weapons and troop types such as Thureophoroi and the Thorakitai who used the oval Thureos shield and fought with javelins and the machaira sword. The use of heavily armored cataphracts and also horse archers was adopted by the Seleucids, Greco-Bactrians, Armenians and Pontus. The use of war elephants also became common. Seleucus received Indian war elephants from the Mauryan empire, and used them to good effect at the battle of Ipsus. He kept a core of 500 of them at Apameia. The Ptolemies used the smaller African elephant.

Philip II used a dense military formation called what?