Genesis 27:40

40.By thy sword shalt thou live, and shalt serve thy brother. It is to be observed that events are here predicted which were never fulfilled in the person of Esau; and therefore, that the prophecy is concerning things at that time far distant. For Jacob was so far from having obtained dominion over his brother, that on his return from Padan-aram, he suppliantly tendered him his obedience; and the breaking off of the yoke which Isaac here mentions, is referred to a very remote period. He is therefore relating the future condition of Esau’s posterity. And he says first, that they shall live by their sword: which words admit a twofold sense, either that, being surrounded by enemies, they shall pass a warlike and unquiet life; or that they shall be free, and their own masters. For there is no power to use the sword where there is no liberty. The former meaning seems the more suitable; namely, that God would limit his promise, lest Esau should be too much exalted: for nothing is more desirable than peace. The holy people also are warned that there will always be some enemies to infest them. This, however, is a very different thing from living by his own sword; which is as if he had said, that the sons of Esau, like robbers, should maintain their security by arms and violence, rather than by legitimate authority. A second limitation of the promise is, that though armed with the sword, he should still not escape subjection to his brother. For the Idumeans were, at length, made tributary to the chosen people; (49) but the servitude was not long continued; because when the kingdoms were divided, the power by which they had held all their neighbors in subjection and fear, was cut off; yet the Lord would have the Idumeans brought into subjection for a short time, that he might furnish a visible demonstration of this prophecy. As to the rest of the time, the restless and unbridled liberty of Esau was more wretched than any state of subjection.