What is the significance of the word "Achar" (as opposed to "Acharei")?
Rashi: "Achar" means immediately afterwards, whereas "Acharei" would mean a long time afterwards. 1 Consequently, the following dialogue took place immediately after Avraham's victory (as we will now explain).
This follows R. Yudan (in Bereishis Rabah 44:5). Rav Huna (ibid.) says the opposite.
What is the connection between the forthcoming dialogue with HaSh-m, and Avraham's battle with the four kings?
Rashi: Avraham (like Yaakov his grandson after him 1 ), was worried a) that, having experienced a great miracle in defeating the four mighty kings, had used up all his merits 2 and was no longer worthy of Divine Protection; and b) that he would be punished for killing so many people. Therefore, HaSh-m reassured him that he had nothing to fear, because HaSh-m would continue to protect him and there was much reward still coming to him.
Targum Yonasan: HaSh-m reassured him because he was afraid a) that he had received all his reward in this world and had therefore forfeited his portion in the World to Come, and b) that the relatives and friends of the four kings world band against him to avenge their defeat. 3
Ramban: It was also to allay his fears that he would die childless. Therefore, HaSh-m added that there was still an abundance of reward in store for him, for constantly going with HaSh-m (as the following Pesukim will clarify).
Mishnas R. Aharon (Vol. 1, p. 119): How could he fear that due to Mesiras Nefesh for Kidush HaSh-m and saving Lot, his merits would be reduced? A miracle that was Kidush HaSh-m and revealed Emunah was a great reward. Therefore he was worried. Ohr Yechezkel writes (Midos, p. 171), Avraham's Midah was Chesed. He felt Hakaras ha'Tov for all he received. He was worried that he became a taker, and lost all of his reward.
Ramban, Rashbam and Seforno all offer this explanation as well.
According to another explanation in the Midrash, HaSh-m assured Avraham that the sons of the defeated kings would not attack him to avenge their fathers' blood. What then does this verse mean?
Maharal (Gevuros Hashem Ch. 6, p. 37): Avraham was afraid he had sinned [though the war] and he would be vulnerable to counter-attack. Hash-m assured him, "Do not fear [a war, for] I shall protect you;" and in regard to sin, "your reward is very great" and you certainly have not sinned. This idea is expressed in the Tefilah, "Magen Avraham." (Refer to 12:2:2.1:1*. Avraham was the first of the Avos. A foundation must always remain firm and invulnerable to outside forces; therefore all of the other nations were unable to harm him.
According to yet another explanation in the Midrash, HaSh-m assured Avraham that he had not used up his reward in Olam ha'Ba [through the miraculous victory in Olam ha'Zeh]. What then does this verse mean?
Maharal (Gevuros Hashem Ch. 6, p. 37): The word "Magen" can also be read as the Aramaic word "Magan," "for free." Hash-m assured him, "Do not fear" that you have used up your reward, for your miraculous salvation was for free (dividends), while your reward (the principal) awaits you in Olam ha'Ba. Chazal (Pe'ah 1:1) list four Mitzvos that earn such a double reward, and Avraham fulfilled them all.
Why does the Torah call this prophecy a "Machazeh"?
Ramban: Until now, Avraham's had only experienced prophecy in nighttime visions; but now he merited communication from HaSh-m by day.
Maharal (Gevuros Hashem Ch. 22, p. 96): The Bris Bein ha'Besarim (later in this Perek) was made with Midas ha'Din (strict justice), so that the covenant regarding Eretz Yisrael could not be revoked due to sin. "Chazon / Machazeh" is the term for prophecy that is appropriate for Midas ha'Din.
Why does the Torah add the word "Me'od (very great)"?
Seforno: To teach us that Avraham would receive reward, not only in this world, but also in the World to Come. 1
QUESTIONS ON RASHI
Rashi writes: "HaSh-m told Avraham, ... 'I shall protect you' - You shall not be punished for all of the people you killed [in battle]." Why didn't Rashi explain this already in regards to the first phrase, "Do not fear, Avraham"?
Gur Aryeh: If Avraham feared punishment for having killed his enemies, he would not have done so in the first place. Avraham realized that if he was victorious through a miracle, his enemies must have been completely wicked, and he would not be held accountable. Rather, at first Avraham was only worried that his Heavenly reward would be depleted due to the miracles he experienced. Once HaSh-m assured him that it had not, he worried this meant his victory was not miraculous, and then perhaps his adversaries were not all that wicked. Therefore, Hash-m added, "I shall protect you."
Rashi writes: "'After' - [After HaSh-m performed] this miracle for him." It is clear that this prophecy is connected to the war that preceded it. According to one explanation in the Midrash, HaSh-m assured Avraham that none of the enemies had been righteous. What then does this verse mean?
Maharal (Gevuros Hashem Ch. 6, p. 37): The four kings aimed from the outset to fight against Avraham and his unique mission. They represent four extremes; they opposed Avraham who was the mainstay of the world, alone. 1 HaSh-m told Avraham, "Do not fear," for the enemy soldiers were as thorns in a garden. "Your reward is very great;" i.e. all of your deeds will turn out for the best [meaning, even when you do not intend it, I shall protect you from causing harm].
In other words, Avraham was akin to the center in respect to the four outer directions. Avraham directed mankind towards its true purpose, and defeating the four kings advanced that goal and in itself deserved reward.