main guard
/ˈmeɪn ˌɡɑːd/noun
Military. A body of troops constituting the chief guard, especially a body of cavalry posted on the wings of a camp towards the enemy; a guard in a fortress taking custody of disturbers of the peace, etc.
Origin
Mid 17th century; earliest use found in Henry Hexham (fl. 1601–1650), soldier and author. From main + guard.