Les Hammond, I agree that ‘out of the blue’ events affected historical battles, and if you are going to reproduce that, you may need chance elements which significantly affect one side or the other. On the other hand, they may not be a good ‘game’ experience.
Suppose, playing solo, you draw a ‘wildcard’ on the first turn that says ‘The generals decide not to fight. Pack up your figures and don’t play again for another week’. That may be historical; there are many occasions when armies got close to a battle, but nothing happened. However, I suspect that you’d ignore such a ‘wildcard’ (or not have it at all). Equally, if I’ve travelled an hour or so for a game, and in the first or second turn there is a chance event that means the rest of the game is an exercise in watching my army be destroyed, it may not be something I want to repeat, however historical.
As Chris Pringle says, you need a balance between predictability and luck. Too much of either and you haven’t a good game (unless you like playing chess or roulette).