Albert Barnes Commentary Amos 5:9

"that bringeth sudden destruction upon the strong, so that destruction cometh upon the fortress." — Amos 5:9 (ASV)

That strengthens the spoiled - (Literally, “spoil” English margin) probably, “makes devastation to smile on the strong.” The “smile,” in anger, attests both the extremity of anger and the consciousness of the ease with which the offense can be punished.

They were strong in their own strength; strong, as they deemed, in their “fortress”; “strong with an evil strength, like one frenzied against his physician.” But their strength would be weakness. “Desolation,” when God willed, would “smile at” all that they accounted “might,” and would “come against the fortress,” which, as they deemed, “cut off” all approach.