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

:  contrary to

:  in competition with

:  in preparation for the time when