What literary device is used in this sentence ... "What the common soldiers did not realize was that the commanders for both sides were engaged in a large scale chess match in which they were the pieces

Personification.

Explanation:

The Soldiers are being personified as chess pieces in the passage. What the commanders are referencing is that they are in an intense battle and have little care or thought for the soldiers.

They are merely the pieces in which the Commanders Strategy is carried out through. The Soldiers have no idea that this is happening.