حَتَّى إِذَا أَلْقَتْ يَدًا فِي كَافِرٍ وَأَجَنَّ عَوْرَاتِ الثُّغُورِ ظَلَامُهَا
Until the sun, as though, placed a hand under a covering, and the night’s darkness concealed the exposed places along the frontier.
Kāfir here carries the lexical sense of a covering—darkness—not the later religious designation ‘unbeliever.’ Ajanna also means to cover or conceal. ʿAwrāt al-thughūr refers to exposed or vulnerable gaps in frontier positions.