noun
punitive force; expeditionary force
A military unit dispatched to subjugate, suppress, or punish a rebellious or hostile group. Often used in historical, fantasy, or news contexts.
政府は反乱を鎮圧するために討伐軍を派遣した。
The government dispatched a punitive force to suppress the rebellion.
討伐 is the act of subjugation or suppression; 討伐軍 specifically refers to the military force carrying out that act.
Compound of 討伐 (subjugation, suppression) and 軍 (army, force). The exact historical derivation is uncertain, but the word is transparently formed from these elements.