What made the Aramean army flee its camp outside Samaria?

Answer: The sound of a great army

The Lord made the Arameans hear chariots, horses and a great army, so they thought Israel had hired Hittite and Egyptian kings and fled.

“For the Lord had caused the Arameans to hear the sound of chariots, horses, and a great army, so that they said to one another, “Look, the king of Israel must have hired the kings of the Hittites and Egyptians to attack us.””— 2 Kings 7:6 (BSB)