Under what conditions is standing firm against the enemy obligatory?

Primary text

Standing firm is obligatory under two conditions. Firstly, the disbelievers must not outnumber the Muslims by more than double their number. If they exceed that ratio, fleeing is permissible according to the divine decree that lightened the obligation: Now Allah has lightened for you, and He knows that there is weakness among you. So if there are from you one hundred patient [men], they will overcome two hundred (Quran 8:66). This passage is interpreted as a command rather than a factual report because if it were merely factual, the reduction in the required ratio from one in ten (as previously understood based on a narration from Ibn Abbas) to one in two would not constitute a 'lightening' (takhfif), and Allah's reports are true and never fail to occur as reported.