A series of (un)lucky events
Joe is on holiday in Saudi Arabia where he accidentally saves the life of the Sheik who nominates Joe his sole heir. He marries the daughters of his rich neighbours and become sovereign of good part of the middle east.
While celebrating his marriage with the King of England in Las Vegas (hey, it's legal now), he manages to double his fortunes. The King convinces Joe to use the money to concur for American presidency (his dream has always been to re-acquire the old colonies). Joe wins.
During a visit to the Norad, Joe falls on a button and starts a nuclear attack. Trying to help the technicians to stop the launch Joe actually hampers them. It's a disaster. Half the world is destroyed in the nuclear holocaust. The people in the Norad (including Joe) survive.
Joe is still King of Arabia and with the available resources manages to buy/conquer the rest of the world (there is not much left), in a bid to destroy any nuclear weapons left and prevent a second tragedy (the world is full of idiots after all).
Edit: a variation of the (un)lucky promotion is narrated in the movie Idiocracy (which I discourage you from watching): an average Joe is hibernated for a military experiment. The project is shut down in a hurry so they forget to wake Joe up. He ends up in a distant future where everybody is an idiot because they spend all time watching TV. At the end the protagonist becomes president of the world because he's now the most intelligent person...