MILITARY FORCES OF THE GRECOBACTRIAN KINGDOM

Alexander and Seleucus I both settled Greeks in Bactria, while  to keep their Macedonian settlers farther west. Greek garrisons in the satrapy of Bactria were housed in fortresses called phrouria and at major cities. Military colonists were settled in the countryside and were each given an allotment of land called a kleros. These colonists numbered in the tens of thousands, and were trained in the fashion of the Macedonian army. A Greek army in Bactria during the anti-Macedonian revolt of 323 numbered 23,000.

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The army of the Greco-Bactrian kingdom was then a multi-ethnic force with Greek colonists making up large portions of the infantry as pike phalanxes, supported by light infantry units of local Bactrians and mercenary javelin-wielding Thureophoroi.

 The cavalry arm was very large for a Hellenistic army and composed mostly of native Bactrian, Sogdian and other Indo-Iranian light horsemen. 

Polybius mentions 10,000 horses at the Battle of the Arius river in 208 BCE. Greco-Bactrian armies also included units of heavily armored cataphracts and small elite units of companion cavalry. The third arm of the Greco-Bactrian army was the Indian war elephants, which are depicted in some coins with a tower (thorakion) or howdah housing men armed with bows and javelins. This force grew as the Greco-Bactrian kingdom expanded into India and was widely depicted in Greco-Bactrian coinage. Other units in the Bactrian military included mercenaries or levies from various surrounding peoples such as the Scythians, Dahae, Indians and Parthians.

 The Greco-Bactrian rulers  had war elephants,which were used in warfare against rivals. An early source mentions the presence of war elephants in Bactria during the reign of King Euthydemus I ( 206 BCE) . During his eastern campaign, King Antiochus III the Great tried to take back the provinces of Bactria and Parthia. As a result of this campaign, Euthydemus and Antiochus signed a peace agreement, in which the Greco-Bactrian King had to give all his war elephants. 

Also, archaeological finds testify to the presence of war elephants in Bactria. One of the Greco-Bactrian artifacts that have been found is a horse's decorative plague, which shows a turreted war elephant .One of the warriors is wearing a distinctively Greek helmet. The other warrior wears a thick chevelure. Both carry long spears. In the Hellenistic Times, towers were put on elephants.It is safe to assume that war elephants were used for a long time, since there is reference to them in the much later Milinda Panha.
 Usually, the elephants were placed at the front of the troops.The aim for them was to break the enemy line.If the enemy resisted, the Greco-Bactrian phalanx would open to give way to the animals and after they retreated, the different pedestrian and cavalry groups would enter the battle.

Sources : Ancient History Encyclopedia ,Wikipedia, idosi.org , Battles of the Ancients