on guard against

: one assigned to protect or oversee another: such as

: a person or a body of persons on sentinel duty

: troops attached to the person of the sovereign

: to protect an edge of with an ornamental border

: to protect from danger especially by watchful attention : make secure

: to stand at the entrance of as if on guard or as a barrier

: in opposition or hostility to

: not in conformity with : contrary to

: in competition with

: in preparation for the time when