Jamieson, Fausset and Brown on Isaiah 11:13

13. envy ... of Ephraim ... Judah--which began as early as the time (Jud 8:1; 12:1, &c.). Joshua had sprung from, and resided among the Ephraimites (Nu 13:9; Jos 19:50); the sanctuary was with them for a time (Jos 18:1). The jealousy increased subsequently (2Sa 2:8, &c.; 19:41; 20:2; 3:10); and even before David's time (1Sa 11:8; 15:4), they had appropriated to themselves the national name Israel. It ended in disruption (1Ki 11:26, &c.; 1Ki 12:1-33; compare 2Ki 14:9; Psa 78:56-71).

adversaries of Judah--rather, "the adversaries from Judah"; those of Judah hostile to the Ephraimites [Maurer]. The parallelism "the envy of Ephraim," namely, against Judah, requires this, as also what follows; namely, "Ephraim shall not envy Judah, and Judah shall not vex Ephraim" (Eze 37:15, 17, 19).