Parshas Mattos - When Not to Keep Your Word

Parshas Mattos begins with a survey of the laws of oaths addressed to the tribal leaders. 

Why specifically the tribal leaders?

While Mattos may begin with the laws of oaths and vows, it concludes with the leaders of two tribes requesting that they settle the other side of the Jordan and committing to go and fight with the other tribes until they succeed in their wars. 

Both times a similar phrasing "kol hayotze mepif taaseh" "that which comes from your mouth you shall do" is used. 

But while Avraham bound Eliezer and Yaakov bound Yosef with oaths, Moshe asks no oath of them. He simply tells them that they must keep their word and that there will be consequences if they do not.

Indeed there are two tragic oaths that will later follow, by Yiftah, who vows to sacrifice whatever comes to meet him on his return home, and Shaul, who vows that he will kill anyone who breaks his pledge to eat nothing until the enemy is defeated only to have his son Yonatan unknowingly eat some honeycomb. 

Thus Mattos begins by addressing the laws of oaths and pledges, Shuvot and Nedarim, to the tribal leaders. In pagan societies, tribal leaders wielded total power based on their personal prestige and honor, but in a Jewish society, oaths are taken to G-d, and there are times when they may be undone. 

Mattos emphasizes the importance of keeping your word but it also lays out leniencies defining when vows and oaths may be set aside. While Mattos addresses more familial religious obligations, Yiftah's vow which potentially put him in the position of engaging in human sacrifice, was obviously illegitimate.

The more important lesson is that tribal elders do not operate purely on honor, but on the word of G-d.