Numa

But if they forbade it or withheld their consent, neither soldier nor king of Rome could lawfully take up arms. War had to begin with their verdict that it was just, and the ruler, on receiving this verdict, must then deliberate on the proper way to wage it. And it is said that the dreadful disaster which the city experienced at the hands of the Gauls was in consequence of the illegal treatment of these priests.

For when the Barbarians were besieging Clusium, Fabius Ambustus was sent from Rome to their camp to bring about a cessation of hostilities on behalf of the besieged. But on receiving an unseemly answer, he thought his office of ambassador was at an end, and committed the youthful folly of taking up arms for the Clusians and challenging the bravest of the Barbarians to single combat.