"Parshat Lech Lecha (Genesis 12:1) begins with HaShem commanding Abram to travel to "the land that I will show you," and dwell in the land of Israel. It is there that Abram experiences a metamorphosis to Abraham after going through a number of steps documented in the Parsha. Ultimately, it is Abraham who will inherit the land of Israel for himself and his descendents.
In order to qualify for the guarantee of the land of Israel, Abram was put through two covenants. The first covenant (Genesis 15:9-17) is the covenant "between the parts", which Rashi (15:6) understands represents the responsibility of bringing karbanot (sacrifices), and the second (17:9-14) is the covenant of circumcision. Upon the completion of the covenant of circumcision, Abram's name is changed to Abraham (and Sarai to Sarah) and he is promised to have a nation great in number and quality.
Why were these covenants necessary when Abram had already taken possession of the land (13:17) after settling the dispute with Lot? In addition, following the casual acquisition of "take a walk on the land" (Tractate Baba Basra 100a, commenting on 13:17), Abram conquers the land from the kings, as far as Damascus, in his desire to save his nephew (14:15).
One approach might be that the covenant between the parts and the covenant of circumcision provide different rewards that are necessary even after taking legal possession of the land. Having just battled the kings, Abram realizes that although he currently owns the land, perhaps at some future date, other nations will come to conquer the land and take it away from his descendants. If the acquisition of the land of Israel was solely based on the tour Abram took (13:17) or of the conquest (14:15), then other nations too can gain possession of Israel as "spoils of war". The Torah (Numbers 31:9-11) records that conquest of war enables acquisition by the winning nation.
To safeguard against this possibility, Abram asks HaShem (Genesis 15:8 and Rashi there): With what merit will my descendants maintain possession of this land? HaShem responds with the covenant between the parts that concludes with the promise (15:18-21), in the merit of the karbanot (Rashi, 15:6); that the children of Israel will be rewarded with the entire land of Israel from all ten nations that currently reside there. The land of Israel will host the House of HaShem, the Temple, and the sanctity of that edifice deserves the stability of single ownership throughout the ages, although, it is intended as a "house of prayer for all the nations."
Since the covenant between the parts enabled Abram to receive the physical aspects of the land, only Abram needed to participate in order for future generations to receive it. After Abram completed his pass between the parts, HaShem consumed the separated animals with fire, and the covenant was consummated. Abram's performance and actions gave the children of Israel an eternal lease on the land of Israel. There would be no need for the descendants to perform this covenant.
This is in direct contrast to the requirements of the covenant of circumcision. HaShem commands that the covenant of circumcision must be repeated throughout the generations, as an assurance that HaShem will always be involved with the children of Israel. This will manifest itself through a special relationship between HaShem and the land of Israel, as Rashi explains the phrase "for a permanent possession" (17:8). This is indicated also on an individual level in that Abram now changes into Abraham and as Rashi (17:1) comments on "And you will be complete" that now he will have full control over all the aspects of himself.
Extrapolating this notion to the land itself, the covenant of circumcision ensures for his descendants that in addition to "possessing" the land (through the covenant of parts), they will now will be "in control" of the land due to HaShem's complete guidance and overt involvement from this point on. "A covenant of love and a covenant of the land via this commandment" of Bris Milah (Rashi, 17:2).
We see that there is a distinction of levels between what the covenant of the parts and the covenant of circumcision were to accomplish. While it is true that Abram was promised a great nation, "and afterwards they will leave with great wealth" (Genesis 15:14), by the covenant of the parts, it is not until the covenant of the circumcision that "And I will be for them as a G-d" (17:8), that is with great spiritual "wealth".
This is indicated by the different metaphors used to indicate that Israel will be a great nation. Abram (13:16) is blessed quantitatively with many descendants comparable to the dust, which is not unique by itself. The descendants promised to Abraham, however, were compared to stars (15:5), which are unique and have quality. The physical reward is already promised after the covenant between the parts while he is Abram, but the more spiritual reward is given only after the covenant of circumcision is completed, having changed his name to Abraham.
