Gunfight at the OK Corral is an American event regarding the famous shoot-out.
It requires that 
USA owns Tucson, Arizona which is initially owned by 
Mexico. Then it can happen between 1881 and 1884.
It has a mean time to happen of 5 months, which is decreased by 25% after 1882 and again by 5% after 1883.
It gives USA 1 prestige.