Question about diplomacy

[ano]

Imagine you have a NAP until turn 70 and on turn 50 the guy has put AN up, what now?- you have played 50 turns for naught.

Perhaps you were thinking about something when you were signing such agreement, weren't you? Maybe, you should have stated some conditions which could automatically break the agreement (In our situation condition could be not casting FotA or similar spell). And once again, it was a decision you made and so you and you only should be responsible for it. It was your mistake and if it costs you a game then all you may do is be a bit wiser next time.