יהושע, פרק א׳, פסוק י״ד

The historical agreement between the tribes remaining on the eastern side of the Jordan River and the rest of the nation rests on a careful balance of rights, duties, and mutual responsibility. Because the eastern territories were vast and particularly suited for livestock, Moses granted these lands to the tribes after they committed to not abandoning the rest of the people in the upcoming campaigns [ביאור שטיינזלץ]. While their families are permitted to stay behind, resting safely in their new land with their property and a small guard, they avoid the continued wandering that the other families must endure. Consequently, basic fairness and justice demand that the men go out to fight on behalf of their brothers [מלבי״ם].

These men are required to cross the river as an independent military unit, fully armed and well equipped for battle [מצודת ציון, רד״ק, ביאור שטיינזלץ]. Yet, this heavy duty is not placed on the entire male population. Only the elite warriors are selected to go [רש״י]. The specific choice to send the most capable fighters reveals that the impending conquest will be conducted through natural, tactical warfare. It will not be a purely miraculous campaign where the combat skills of the people are irrelevant [מלבי״ם].

As for the nature of the help they provide, the straightforward understanding is that these warriors must actively assist their brothers in all their physical battles [ביאור שטיינזלץ]. However, another perspective deepens the meaning of their participation. In a practical sense, the other tribes do not actually need military reinforcements, because God is their true helper and shield. Therefore, the primary purpose of these eastern warriors joining the fight is moral support. Their presence demonstrates national unity and prevents any feelings of fear or abandonment among the rest of the nation [חומת אנך].