God is presented as an archer. God is thus depicted as a man, which is not uncommon in the Bible. (Scholars use the term anthropomorphic to describe this phenomenon.)
The opening line of this verse employs simile as a figure of speech, i.e. a way of comparison. God bent his bow like an enemy. Here, the simile is significant, because it avoids a description that terms the Lord as an outright enemy. If he were outright their enemy, there would be no hope for repair and reconciliation.1
If the Lord's bow represents the totality of military force employed against Jerusalem, then this is an example of synecdoche - a figure of speech in which the whole is represented by its part.
4 Il a tendu son arc comme un ennemi; Sa droite s'est dressée comme celle d'un assaillant; Il a fait périr tout ce qui plaisait aux regards; Il a répandu sa fureur comme un feu sur la tente de la fille de Sion.