Behold, they shall surely gather together, etc.; rather, behold, should they gather themselves together; i.e. should enemies collect and threaten thee with harm, be assured that the attack is not by me—not my doing—and that, therefore, it will come to nought.
All those who gather together against thee shall fall—i.e. stumble and be overthrown—through striking against thee. The rendering of the Authorized Version, "for thy sake," is quite indefensible.