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Wizards are in a nuclear arms race to attain the wish spell - plain and simple. Upon reaching 17th level, the only sane thing is to pour all of one's time and money into creating a wish spell. The next step is to use your new power to hunt down and destroy all other existing copies if the spell - and kill or incapacitate the creators of those spells.
Of course, containment of the wish spell would be impossible. Eventually, someone else would figure it out, or some extra-planar being will drop by with the power. Magicians would have to form alliances to protect themselves, and as the power of opposed alliances grew, a stalemate like the Cold War would occur. A wizard couldn't remain neutral in this conflict for long. They'd be forced to choose sides.
If the stalemate faltered, however, opposing sides would wish each other out of existence in a flash. Wishes could even be delayed with dead-man switch type technology to obliterate the other side even if the original wisher was wishified. (Which is kind of wishy-washy.)
So, assuming that the magic-users didn't devastate the planet and themselves, they would operate in Magic Unions formed around an agenda - usually the survival and increased power of the Union and its near immortal founder - the "Wish Master" (ick, probably need a better name for that.) Operating out in the open would be the last thing they would do as they would want to be seen and detected by rival Unions as little as possible. They would operate much like a cabal.
I can't see that running countries would be in their interest. At least, not out in the open. It would be much safer to pull stings from the dark. Participating openly in military campaigns would even be iffy. High powered magicians would spend much of their time in study and research. I kind of see them as above (or below, if you will,) affairs of state. They'd also want to have a way to get things done in a non-magical way - thus they would be assassins or ninja or some other stealthy, elite force that does not rely, or emit, magic.
So, it's magical cabals fighting each other in the dark over powers that can rip holes in reality, all the while, pulling secret strings in the mundane governments to futher their knowledge and survival. Well, at least that is the way I see it. Of course, it depends on how the rules are tuned.
What do you see?