What commandments did Reuben and Gad break?

Question

The Sources criticize the tribes of Reuben and Gad for two things: Not settling in the Land of Israel and placing their cattle above their children. [Midrash Tanhuma, Matot, Siman 7; Numbers Rabbah 22:9; Proverbs 20:21] They were punished by being the first of the Ten Tribes to be exiled.

1-They did not settle in the Land of Israel because the land east of the Jordan was better for their cattle. They got permission from Moses, in exchange for fighting with the other tribes in the frontline, which they did for seven years. Whether or not a Jew is commanded to live in the Land of Israel is still a matter of dispute. Naḥmanides says yes and Maimonides says no, according to the lists of 613 they left us. Both sages acted on their beliefs. When Maimonides was forced out of Muslim Spain, he lived only very briefly in Israel, then moved to Egypt, did all his work there and died there. When Naḥmanides was forced out of Catalonia, he went directly to Israel, and worked and died there.

2-They said: "We will build here enclosures for our flocks and towns for our children." [Numbers 32:16], and Moses gently rebuked them by reversing the order: "Build towns for your children and enclosures for your flocks". [Numbers 32:20,24] But there is no commandment to build dwellings for your children before you build enclosures for your cattle. And they never said they were going to start with the cattle pens, they only mentioned them first to emphasize that they were settling there because of their cattle.

So what commandments did Reuben and Gad break?

One could argue that the Torah says: "Be sure to keep the commandments… Do what is right and good in the sight of God." [Deut. 6:17-18] The Talmud says the repetition means that you must sometimes go beyond what is strictly commanded. Perhaps Reuben and Gad did not do that. But it's a very subjective matter.

Answer 1

The way the question is stated - is somewhat misleading. Bnei Reuven and bnei Gad were not exiled after they chose to live east of Jordan. However, their choice to be separate from other tribes created a number of problems, which resulted in sins, caused the decline of their society, and perhaps resulted in early exile many years later since they were also the first frontier of defense against Ammon, Moav, Edom, and Aram.

There is a comment of Tur, haAroch on Bereshit 30:11,

ותאמר לאה בגד. י"מ על שם שבני גד היו ראשונים שבגדו לקחת נחלה בארץ ישראל כדכתיב ויגשו בני גד ובני ראובן. Blockquote The meaning of the root בגד - betray. There are comments that bnei Reuven and bnei Gad were first to betray (joint tribal acquisition of Eretz Israel)

Sefer Yehoshua 22 describes a bigger problem, which resulted from settlement of Bnei Gad and Bnei Reuven east of Jordan. They realized that their descendants may not want to go to Shilo for festivals and have connection with other tribes. They started building an altar as a reminder of tribal connection west of Jordan. The building almost sparked the tribal war averted by Pinchas.

Finally, the mussar seforim state that absence of fathers east of Jordan river during the time of conquest negatively affected the children, so that they grew up being more cruel to others. The accidental killings east of Jordan were more frequent than in Eretz Israel, which had a higher population density. Rashi on Bamidbar 35:14.

In other words, one may make a halakhically acceptable decision which may, neverthelss, yield bad and difficult consequences. Bnei Reuven and Bnei Gad did not violate halakha, but their decision had bad consequences.