is confederate with - rather, is encamped upon the territory of Ephraim [Maurer], or better, as Rezin was encamped against Jerusalem, “is supported by” [Lowth] Ephraim, whose land lay between Syria and Judah. The mention of “David” alludes, in sad contrast with the present, to the time when David made Syria subject to him (2Sa 8:6).
Ephraim - the ten tribes.
as ... trees of ... wood - a simultaneous agitation.