Genesis 27:40

And by thy sword shalt thou live
By what he could get by it; his land being so poor that he could not live upon it, but must be obliged to such methods for a livelihood; or his country being surrounded with enemies, his posterity would be obliged to defend themselves by the sword, and other weapons of war: and shalt serve thy brother;
which is the sense and language of the oracle, ( Genesis 25:23 ) ; and which Isaac perhaps now remembered, and had a clear understanding of it, and delivers out his prophetic blessing agreeably to it: and it shall come to pass, when thou shalt have the dominion;
not over the Israelites, the posterity of Jacob, which the Edomites, Esau's posterity, never had; but when they should get a greater degree of strength, power, authority, and dominion in the world: that thou shalt break his yoke from off thy neck;
the Edomites should revolt from the posterity of Jacob, and shake off the yoke of bondage and subjection they had been long under; as they did in the times of Joram, king of Judah, and set up a king of their own, and continued in such a state of freedom a long time, see ( 2 Kings 8:20-22 ) .