Or HaChaim on Genesis 25:23/אור החיים על בראשית כ״ה:כ״ג

1 ויאמר ה׳ לה, G'd said to her, etc. Although this verse has been explained by many there is still no truly satisfactory answer. G'd responded to her fear that she would miscarry by explaining to her that there was nothing medically wrong in her womb but that she was carrying two instead of merely one fetus. Whereas normally, when a mother expects twins, the two fetuses get along inside the womb (compare Song of Songs 7,4), in Rebeccah's case they did not. Hence her feeling of being crushed. She did not merely carry twins but each one was destined to become a nation with very different characteristics from one another. Not only would each one be a king in his own right, but these respective nations would endure for thousands of years. All of this would not have resulted in her having a feeling of being crushed were it not for the fact that these twins did not conform to the usual pattern of twins. When G'd explained that ממעיך יפרדו, "they will be totally separate already while still inside of you," this meant that they would not only be separate inside of Rebeccah but their being separate would continue after they were born. At any rate, G'd reassured Rebeccah that there was nothing wrong with her womb.

2ולאום מלאום יאמץ, "and one nation will be stronger than the other, etc." This means that there is an additional factor preventing these two peoples from dwelling together in harmony; each one will derive its strength from the other (the defeat of the other). We find an example of this concept in a statement in Megillah 6 that the city of Tzor attained its true prominence only through the fall of Jerusalem. Seeing that each nation therefore anxiously awaits the downfall of the other, there is no hope that they will live together in brotherly harmony.

3ורב יעבד צעיר, "and the senior one will serve the younger." None of the many commentaries on this expression seem to do justice to the words. Our approach will solve the problem. Not only will one of these two nations attain its respective grandeur at the expense of the other, but the nation which is defeated will wind up in servitude to the respective victor. The victor will not content himself with having vanquished his antagonist and deprived him of his assets, but he will oppress him to boot. This will further inhibit any chance of reconciliation and will increase the degree of mutual hatred. This is what the Talmud (Ketuvot 72) had in mind when it stated that a human being and a snake cannot live together in cramped quarters. The words do not mean that Esau and Jacob respectively will live in such a dependence on each other; the meaning is that whoever will be the senior one of these two nations during different periods of history will relate to the other as a tyrannical overlord.