In Parsha Matos, Sh’vatim Gad and Reuven approached Moshe Rabbeinu regarding their desire to graze their flocks and settle their families on the East side of the Yarden. To this, Moshe Rabbeinu replied, “Shall your brothers go off to war, and shall you sit here?” (Sefer Bamidbar, Perek 32, posuk 6)
Moshe was quite angry at the two Sh’vatim. He was concerned lest Gad and Reuven would avoid taking part in the wars for Eretz Yisrael, that other Sh’vatim might follow suit and B’nai Yisrael might be condemned to wandering in the desert another 40 years.