When appointing Joshua, Moshe makes a request, and God (while seeming to agree) adjusts it in a small but critical way.
Moses' talks of a leader who will "Lead them out and bring them back," meaning that the national leader will have the power and responsibility to declare and wage war.
God, however, tells Moses that Joshua
"Shall stand before Elazar the priest ... BY HIS WORD shall they go out and BY HIS WORD shall they come in."(v.21).
Moshe sees the national figurehead as the person who will make the decisions of war and that he, as the commander in chief, can make that decision alone. God informs Moshe that Joshua cannot decide exclusively; he must consult with the High Priest, Elazar, before taking the nation to war. (The High Priest is to consult God via the prophetic oracle called the "Urim".)
Why do we need this detail here? Because God is evidently changing the status quo. Moses led alone. He was the prophet and the king, the commander and the lawgiver. He was the ultimate leader. He had no legal recourse to consult with Aharon the High Priest, nor any of the tribal heads. Moshe took his issues directly to God.
But leaders of the future will not be like Moshe. Now authority needs checks and balances, decisions need to made in consultation; there needs to be oversight. Here is a new system for a post-Moses world.
Halakha reflects this innovation when it rules:
"The king cannot take his people out to a non-defensive war without the approval of the High Court of seventy one scholars." (Rambam. Hilchot Melakhim)
There was no prophet like Moses. But ever since then, a national leader needs to consult and even at times, defer to an alternative authority. We must never imagine that a single individual has all the answers , nor may we allow them to hold exclusive and supreme authority.