And he shall stand before Elazar the Kohen, who shall inquire for him by the judgment of the Urim before Hashem; at his word they shall go out and at his word they shall come in, he and all the children of Yisrael with him, and all the congregation.
Torah Temimah: Yerushalmi Sanhedrin 2:5
Sefaria — Yerushalmi Sanhedrin 2:5
English: It was taught: the king may take [the people] out to an optional war only by the word of a court of seventy-one, as it is said “at his word they shall go out and at his word they shall come in.”
Constitutional limit on royal war.
Optional war requires Sanhedrin authorization under this verse’s language.