rear guard; second reserve
Military term for troops positioned at the rear for protection or as a secondary reserve force.
The rear guard unit prevented the enemy's pursuit.
The commander deployed a second reserve to cover the main force's withdrawal.
second reserve service; military reserve force of ex-servicemen
Compound of 後 (rear) and 備 (equip, provide), literally 'rear preparation'. The exact historical derivation is uncertain, but the term is used in military contexts.