opposing forces; counter-force unit
Refers to a military or organized unit that is set up to oppose or counter another force. Often used in strategic or competitive contexts.
A counter-force unit was formed to stop the enemy's advance.
対抗馬 is used in elections for a rival candidate, while 対抗部隊 is a military or organizational counter-unit.
部隊 is a general term for a unit or corps; 対抗部隊 specifies a unit formed for opposition.
Compound of 対抗 (opposition, counter) and 部隊 (unit, force). The exact historical derivation is uncertain, but the components are transparent.