Parshas Matos begins with a detailed treatment of the halachos of nedarim - vows. The parshah then turns to the war against Midian, followed by the decision of the shevatim of Reuven and Gad to settle on the east of the Yarden and their subsequent agreement with Moshe to first fight and assist in the conquest of the land on the west, where the other shevatim were to live.

One can ask why the halachos of nedarim appear in Parshas Matos, as they seem to be unrelated to the balance of the parshah, which deals with war and conquest of land. What is the relationship between nedarim and the rest of this week's parshah?

The relationship between nedarim and the balance of the parshah appears to be that of honoring one's commitments, even when the commitments are old and perhaps seemingly irrelevant. Moshe Rabbeinu chastised the tribes of Reuven and Gad for deciding to remain on the east of the Yarden, as it was originally understood that all of the tribes would jointly enter the Land and engage in a communal conquest; the proposal by Reuven and Gad to settle to the east - which was heretofore inconceivable, but now seemed very logical and doable - represented an abandonment of their previous commitment and was seen by Moshe as endangering the nation as a whole.

So, too, the war against Midian was primarily waged in order to fulfill a prior obligation imposed by Hashem in Parshas Pinchas (25:18). Chazal note that Moshe was joyful and alacritous in his fulfillment of the command to wage war on Midian to avenge for that nation’s ensnarement of B’nei Yisroel into sin at Ba’al Pe’or, even though Moshe was told that he would die after the war. (Rashi on ibid. 31:3, from Sifri). Moshe nonetheless acted with alacrity in performing God's command and honoring the imposed commitment to wage the war.

Thus, the preliminary section of the parshah dealing with nedarim serves as an introduction to the general obligation to honor previous commitments, despite the fact that one may no longer view such commitments as relevant or in one's best interests at a later time.

The siman of Parshas Matos is also indicative of this motif, for the siman is "Eival", which refers to Har Eival (Mount Eival), upon which B'nei Yisroel were commanded that they erect large stone tablets inscribed with the Torah or the mitzvos at the time of their entry to Eretz Yisroel (Devarim 27:4). The purpose of these large tablets was to always remind the people that they undertook a covenant with Hashem upon leaving Mitzrayim, and - even though they now had their own land and were successfully settled with all of their needs - they dare never forget the covenant's eternal relevance and obligations.

Why is the theme of honoring previous commitments featured and emphasized in Parshas Matos? Is it not a general concept throughout the Torah?

Chazal note on several occasions (e.g. Bamidbar 27:12, Devarim 3:23) that Moshe Rabbeinu suspected that God would rescind His ban on Moshe's entry to the Land due to various circumstances that indicated to Moshe a possible change in Hashem's plans for him. Why did Moshe think that God would change His mind, so to say?

As was explained in the divrei Torah on Parshas Bamidbar and Parshas Chukas in this series, B'nei Yisroel underwent a major transition in Sefer Bamidbar. They were weaned from being the Dor Deah - a generation living on open miracles - to a nation of conquerors and farmers, in preparation for settling the Land. Many of the open miracles ceased, and the nation needed to exert human effort for military success and had to toil for the acquisition of land and property. This transition was quite difficult, and it served as the basis for many of the nation's low-points in Sefer Bamidbar. Underscoring the changes which transpired was a general change in the manner that God dealt with B'nei Yisroel and His expectations of them. Thus, under what seemed to be a totally new state of affairs, it appeared feasible that some of the "ground rules", limits, or expectations of people would be affected; this is why Moshe Rabbeinu suspected that the ban on his entry to the Land would be rescinded.

This is where Parshas Matos comes in, toward the end of Sefer Bamidbar, the book of transition, to boldly proclaim that every commitment and obligation must nonethless be honored under all - and even totally different - circumstances. Just like a neder is "chal" (takes effect) and cannot be violated unless it is specifically cancelled by a beis din, so, too, obligations previously made must be followed through, despite the most different of circumstances later on. This is Hashem's unchanging Will, and it is the basis of Halacha and our relationship with God.