Since I don't play MP via these forums but with friends, I probably have a very different approach to diplomacy than that which appears, from the statements earlier in the thread, to be the general case here.
Namely the case that diplomacy appears to be about assurances divinely held sacred as if they were a part of the game itself and imposing strict in-game limitations on actions rather than being mutually beneficial agreements that you have to nurture to maintain and, as such, are likely to be broken when one party thinks it is no longer of advantage to itself to follow it and the disadvantages of being known to break an agreement in a particular game are less than the advantages.
I realize I won't change your minds on the NAP issue if what you prefer playing with is a NAP that must be obeyed just as if it was an actual in-game rule (which it is not) - it is a convention of your gameplay, and if that's how you like to play it, that's certainly fair enough, but from a practical perspective it is nothing more than a convention - there's certainly no rational argument for why it
should be that way, and there's no reason to expect your opponents to play by such player created rules unless they've agreed to do so.
In fact, diplomacy where verbal or written agreements between players must be kept no matter the circumstances is in general in games considerably less interesting and presents fewer opportunities to excel at the art of diplomacy than games running a more commonly accepted convention in board and card games featuring intrigue and player elimination: Anything that is agreed between players to be done "now" in the current turn must be done, anything that is agreed for the future is enforced solely by the might and diplomatic capabilities of the players involved. (That is again only one convention and not necessarily the one you'll have most fun with, but it makes for considerably more challenging diplomacy and demonstrates one of the core values of real-life diplomacy: false security.)
If I'm playing with my friends and I agree an alliance, a turn limited NAP, a truce, or any other diplomatic relationship lasting more then the current turn, then I expect my great friend and wonderful player, who'm I'll liberally praise while searching for the right place to knife him in the back come the day I need him no longer, to follow it so long as the advantages of doing so outweigh the disadvantages of not doing so and not a moment longer. Of course, there are advantages to "doing the right thing" and being a "man of your word", so betraying somebody has a significant negative impact on your diplomatic abilities long-term in the same game in most cases, which requires an equally significant advantage to make betrayal worth it... but that's just one aspect of the evaluation of whether to break an agreement or not.
I'll regret it publicly when the nasty deed gets done and appeal to world peace, the international order, or the maintenance of the balance of power... should I be the one betrayed... and I'll explain why it was a necessity to break with the untrustworthy ruthless powermongering bastard, who was clearly setting up his game plan for an overwhelming attack on myself, which I only twarted by preempting him at the last possible moment, sacrificing myself for the good of all.... should I be the one doing the betraying. (Actually, to better sell this idea, I'll of course be buttering up the other players who are not directly affected preferably one or two turns in advance - it risks being counter-betrayed or preempted, but establishing the moral high ground and laying the foundations for general acceptance of your actions amongst those who might tip either way is usually very important in game where diplomacy really matters)
Now, THAT is part of what real diplomacy is about in the sort of games I like to play, including Dominions 3 MP, diplomacy that does not force compliance with your words when you no longer intend to honour them - diplomacy where your ability to
nurture a strategic relationship is as important as your ability to get somebody to sign an agreement with you in the first place.
....So to answer the poll - in the situation as described, I'd break the agreement in a heartbeat unless I had explicitly agreed to a convention of enforcing agreements regardless of what's happening in the game.
Backstab, betray, deceive, and destroy as necessary, aid, selflessly sacrifice, work for the common good when it benefits yourself..... all these make for wonderful moments in diplomacy, even though they do tend to create short-lived bursts of temper.... so long as players don't bring grudges from one game to another or let the actions in one game affect their actions in another. Somebody pulled a clever betrayal of you in one game that you didn't see coming? Good for them, that's a good lesson for you to be more wary in the future and perhaps even more diplomatically aggressive - next time it might be your knife in somebody's back.