DEUTERONOMY 3:18

At that time I charged you, saying, “Hashem your God has given you this country to possess. You must go as shock-troops, warriors all, at the head of your Israelite kinsmen” DEUTERONOMY 3:18 (The Israel Bible™)

The tribes of Reuven and Gad requested to settle on the eastern side of the Yarden in the lands conquered from Sihon and Og, rather than join their brethren on the western side. Moshe granted this request on condition that Reuven and Gad would fight alongside the other tribes in the conquest of the land. Here, Moshe reminds Reuven and Gad of their agreement. And, indeed, when Yehoshua succeeds Moshe and leads the nation into their land, he calls upon these tribes to fulfill their promise. The men of Reuven and Gad answer their call to duty and fight in the army until the battles are over. As in the days of Yehoshua, today the State of Israel calls upon all of its citizens to serve in the Israel Defense Forces. Military service typically lasts for three years, between the ages of eighteen to twenty-one, followed by several weeks of reserve duty each year until the age of forty-five. Similarly, Israel turns to those living outside the State for different, but also important, contributions to its vitality, growth and security.