I'm going to have to disagree with this. Charisma was always important. It's the leadership stat. You want people to look up to and respect you.
You don't have to lead with charisma, you can lead with fear. It's how most warlords, lords, kings, dukes, emperors throughout history have done.
You have to convince lesser warlords to team up with you (and not your competitors) and you need to do this when you're not very strong so it's not just a case of "my army is bigger/more badass than yours".
If you're not strong, you lose and die. Having a nice smile is at the end of the long list of things you need.
Examples of extremely charismatic leaders include Julius Caesar (died 44 BC, was very charismatic on and off the battlefield), Liu Bang (reigned 202 to 196 BC and became emperor specifically because he had better social skills than his chief competitor, who had a larger army and a better sense of tactics) and Ghengis Khan (reigned 1206 to 1227).
Three examples. All of whom were also accomplished warriors. Of those three, only one actually won out because of charisma. Caeser won wars and gained popularity. Khan murdered everyone who stood in his way. If they happened to also be charismatic, that's fine, but they primarily gained their reputations as military men first.
Yes, but even a small kingdom is far too large for the king to do all the murder/threat of murder personally. Or even to directly supervise all the people doing it on their behalf.
No one said otherwise.
Any king, therefore, is going to need some way of securing and keeping the loyalty of subordinate commanders (reputation, family ties, bribery, religious/legal structures, etc.).
Fear. Power. Threats. Brides. The brown stuff rolls down hill. None of which require a charming personality.
Intelligence, Wisdom and Charisma aren't the be all and end all in doing so, but they certainly don't hurt either.
Exactly. The end all be all is power and a willingness or ability to do worse to your enemies than they're willing or able to do to you.
1) Kill current monarch.
2) Put crown on head.
3) Say "Anyone got a problem with this?"
4) Deal with any challengers (see Step 1).
5) Sit on throne and become king.
6) Profit!
7. Sit on the throne long enough and people stop challenging you.
8. Arrange advantageous marriages with other warlords nearby and increase your holdings over generations.
9. Your family sits on the throne long enough and people stop even questioning you.
And now you know how all the monarchies in history were established.