Since the Early Church, this prayer has found several functions: the Collect for a votive mass for Peace, a prayer after the Rogation litany, until Archbishop Cranmer placed it as one of the Evening Prayer collects.  The wording has undergone some slight changes in recent times; it is substantially different in the 1979 Prayer Book but rolled back closer to the original wording here.

O God, the source of all holy desires, all good counsels, and all just works: Give to your servants that peace which the world cannot give, that our hearts may be set to obey your commandments, and that we, being defended from the fear of our enemies, may pass our time in rest and quietness; through the merits of Jesus Christ our Savior. Amen.

The world, the flesh, and the devil are forces that turn us away from God; those are the real threats against whom we need protection, and against whom we must fight.  For, as the Daily Office prayers for Peace express, Peace is not found in avoidance of conflict, but in steadfastness despite conflict; God will defend us from fear so that we can “pass our time in rest and quietness.”  With our trust placed in God’s defense and our hearts set to obey his commandments, we find ourselves on the solid ground of God’s Word, in the footsteps of Jesus, in cooperation with the Spirit.  There, we can withstand the wiles of the world, the flesh, and the devil; there can be found peace that cannot be found anywhere else.