Thou shalt not be afraid for the terror by night. Robbers constituted the chief "terror by night" (see Job 24:14-16; Jeremiah 49:9; Obadiah 1:5); but night attacks on the part of a foreign enemy were not uncommon (So Psalms 3:8; Isaiah 15:1).
Nor for the arrow that fleth by day. Open war is probably intended, not sirocco, or pestilence, or "the arrows of the Almighty" (Job 6:4). The man who trusts in God will be specially protected in the peril of battle.