Chief Hanlon’s mistake was in assuming that the legion will stop with Hoover Dam; people like Caesar don’t stop taking until either someone else stops them, or there’s nothing left to take. If the the dam is lost to the legion; Caesar isn’t going to be satisfied; he’s gonna think “I am completely invincible!” Conquer the rest of the Mojave, and invade the NCR.
Best possible result of that invasion: Ceasar is killed while destroying Junktown, and the legion collapses.
And the worst: the Legion conquers all of New California, and the disaster Hanlon sought to prevent is replaced by an even bigger one.