You should have warn your opponent 1 or 2 turns before, it is usually quite obvious you will be able to win in the next 2 turn if nothing change. So you should ALWAYS give a warning at least 1 turn before.
I do a lot of truce, and I will never break any of them. You will finish with a bad ratings and probably be foed if you do that against me.
I give you an example. I was playing a trench, 3-player on Europe 1914. Player A almost had half the board, so I made a 1 turn warning truce with player B to prevent him winnig. Game wasnt equal, yet player B break the truce and take one capital for me, game being trench and chained I couldnt prevent him winning by objectives next turn.
I find this kind of backstabbing being totally poor play. You'd tell me he won and its the only things that matter? Please never join my game
It's a win by deception and lying, nothing to do with strategy for me.
There is possible exception of course. In any case in escalating for example, but I almost never do truce in escalating game.
But I'd say it like that, if you will win anyway, then wait one turn to give the warning. If you win only because you broke the truce, then it's poor play.
In smthg like 90% of the game I played with a truce , breaking the truce withouth warning would give a determining advantage making the win very likely for the one breaking it, me or my opponent. So if I had followed people who say " if you can win by breaking the truce it would be stupid not do it" I would have break almost all the truce I ever made. Thats wrong. Dont do truce if you plan to break them.