أَخَذْنَ عَلَى بُعُولَتِهِنَّ عَهْدًا إِذَا لَاقَوْا كَتَائِبَ مُعْلِمِينَا
These women have exacted a pledge from their husbands for when they encounter clearly marked enemy units.
Buʿūlah is a plural meaning husbands; ʿahdan names the binding promise obtained from them. Katāʾiba muʿlimīnā describes marked or identifiable battle units.
Idhā lāqaw states the condition of the pledge. The next verse supplies the action being demanded, so the two verses must be read together for the complete sense.