Before rushing into battle against their brothers across the Jordan River, the Israelites choose to pause and send a delegation to investigate the situation. The primary goal of this mission is to confront the tribes about their actions, hoping to inspire repentance and avoid unnecessary bloodshed [מצודת דוד, מלבי״ם]. The logic behind this approach is clear. If the law requires offering peace to an enemy city before attacking, this courtesy must certainly be extended to their own brothers. Furthermore, the Israelites recognize the possibility that the eastern tribes might actually have a reasonable explanation for building their altar.
However, this diplomatic mission is launched only after the Israelite army is fully mobilized and ready for war. This strategic timing is meant to instill fear in the tribes across the river. It sends a firm message that if they reject the delegation's appeal, a fully armed military force is prepared to take immediate action [אלשיך].
To lead this crucial mission, the Israelites select Phinehas, the son of Eleazar the Priest. Although his father is likely still alive at this time, Phinehas is chosen to head the delegation. He is no longer a young man and has perhaps already begun stepping into his father's leadership roles for tasks like this [ביאור שטיינזלץ].
The choice of Phinehas is highly intentional and carries a deep dual significance. The suspicion against the tribes involves two distinct layers: a betrayal against God and a betrayal against the nation. Phinehas is specifically tasked with addressing the religious and spiritual offense against God [מלבי״ם]. At the same time, his presence represents a genuine call for peace, as he has a proven history of bringing peace to the nation. By sending Phinehas alongside military commanders, the Israelites deliver a perfectly balanced message. They offer a sincere opportunity for peace through the priest, while simultaneously presenting a very real military threat through the army leaders, should the peaceful offer be refused [אלשיך].