Jamieson, Fausset and Brown on Isaiah 21:12

12. Reply of the prophet, The morning (prosperity) cometh, and (soon after follows) the night (adversity). Though you, Idumeans, may have a gleam of prosperity, it will soon be followed by adversity again. Otherwise, as Barnes, "Prosperity cometh (to the Jews) to be quickly followed by adversity (to you, Idumeans, who exult in the fall of Jerusalem, have seized on the southern part of their land in their absence during the captivity, and now deride them by your question)" (Is 34:5-7). This view is favored by Ob 10-21.

if ye will inquire, inquire--If ye choose to consult me again, do so (similar phrases occur in Ge 43:14; 2Ki 7:4; Es 4:16).

return, come--"Be converted to God (and then), come" [Gesenius]; you will then receive a more favorable answer.

Is 21:13-17. Prophecy that Arabia Would Be Overrun by a Foreign Foe within a Year.

Probably in the wars between Assyria and Egypt; Idumea and Arabia lay somewhat on the intermediate line of march.