Jamieson, Fausset and Brown on Amos 3:11

11. Translate, "An adversary (the abruptness produces a startling effect)! and that too, from every side of the land." So in the fulfilment, 2Ki 17:5: "The king of Assyria (Shalmaneser) came up throughout all the land, and went up to Samaria, and besieged it three years."

bring down thy strength from thee--that is, bring thee down from thy strength (the strength on which thou didst boast thyself): all thy resources (Pr 10:15).

palaces shall be spoiled--a just retribution in kind (Am 3:10). The palaces in which spoils of robbery were stored up, "shall be spoiled."