( فَإِمَّا ) (So, if): It is a conditional clause consists of two parts. This indicates that every time the condition comes about we are to fulfill the request as  Allah (swt) orders us.

( تَثْقَفَنَّهُمْ فِي الْحَرْبِ ) (Whenever you encounter them in battle): The phrase conveys that every time you come across them in a conflict or war.

( فَشَرِّدْ بِهِم مَّنْ خَلْفَهُمْ ) (So, disperse those behind them): Meaning, make them an example for those who are behind them so that they do not regroup. Just as it is said: A powerful strike frightens others. So, if you encounter them, O Muhammad, you should disperse those behind them, referring to the Jews who were inciting and supplying them with weapons.