﴿ فَإِمَّا تَثْقَفَنَّهُمْ فِى ٱلْحَرْبِ فَشَرِّدْ بِهِم مَّنْ خَلْفَهُمْ لَعَلَّهُمْ يَذَّكَّرُونَ ﴾

“if thou find them at war [with you], make of them a fearsome example for those who follow them, so that they might take it to heart;”

So if fa-immā here the letter nūn of the conditional particle in ‘if’ has been assimilated with the extra mā you come upon them if you find them anywhere in the war deal with them so as to cause those fighters behind them to scatter to disperse by making an example of them and punishing them so that they the ones behind them might remember that they might take heed from their example.