(Devarim 20:1) "when you go out to war against your foes": Scripture here speaks of a non-obligatory (optional) war.
"against your foes": (Remember that) it is against your foes that you are warring.
"and you see horse and chariot": Just as they go out against you with horse and chariot, so, you go out against them with horse and chariot.
"horse and chariot": Now do they have only one horse? Is it not written "a great host"? (The meaning is that) when Israel do the L-rd's will, they are all reckoned as one horse. Similarly, (Shemoth 15:1) "Horse and its rider He has cast into the sea." Was there only one horse? Is it not written (Ibid. 14:7) "And he (Pharaoh) took six hundred choice, chariots, etc."? (The meaning is that) when Israel do the L-rd's will, they are all reckoned as one horse. (Devarim, Ibid.)