The following is meant as an AAR, but as it concerns multiplayer dynamics, I'm pretty sure the Mods of Doom will not accept me posting it in the AAR thread, so I post it here instead.
The Context of Treaties
I think people would do well to consider signed treaties' contexts. There is a spectrum of circumstances under which treaties are signed, and that spectrum is characterized by the amount of duress either party is under.
The extremes of the spectrum are:
1) Two parties recognizing mutual interest, signing a win-win treaty;
2) One party imposing treaty conditions through coercion, either force or trickery, on the other party.
Looking at situation 1: When two parties come to a mutual agreement of their own will, without one party coercing the other, the parties will both want to enforce the terms of the treaty. An example of a situation like this is the agreement between Bavaria, England and Andalucia at the end of CK. We all had a vested interest in dividing the New World between us; we would all win if we stuck to the terms of the agreement and no-one forced anything. The spoils were divided in a way everyone agreed to of their own volition.
Defensive Alliances against a common foe are another example of this sort of treaty: enforcing the terms is in both parties' best interest.
Agreements like these can end, too: circumstances change, common foes are cut down to size. Ideally, the treaty is then ended with mutual agreement. However, one often finds that one party wishes to continue the treaty while another doesn't. To facilitate this, we often include a grace period: "automatic renewal every 10 years" - the 10 years is the grace period, a compromise between both parties of which one wishes the terms to be enforced and the other party doesn't. To maintain credibility it is very important not to infringe on these terms.
Situation 2 is the case where one party imposes its terms on another. In this case there is no mutual interest: one party has an interest in seeing the terms enforced, and does so by coercion. A typical example of this is the peace treaty a victor imposes on the vanquished: the victim has no interest in enforcing the conditions of the agreement; the victim has an interest in getting the victor to back off. In the simplest case, the victor will take something material: provinces, money. In other cases, the victor may impose demands over time: a NAP, or war reparations over time. The vanquished may agree to demands like these under duress. The main enforcing power of such a treaty's conditions is not mutual will and diplomatic credit, but the victor's worldly power. After all, once the roles are reversed, the previously-vanquished-now-victorious can just force the previously-victorious-now-vanquished to reverse the treaty conditions.
In situation 2-cases, the victor's word is more important than the vanquished's. After all, once the vanquished accedes, he is at the victor's tender mercies: the vanquished depends entirely on the victor's word, whereas the victor still has his force of arms to enforce conditions. An example: All the Russias fights Komnenid Rome, and defeats the Roman armies in the field. Both parties agree on a treaty where Rome cedes the Romanian Black Sea coast and the Crimea to Russia, in exchange for a 50-year NAP that allows the Romans to focus on the Croatians. If Russia breaks its word and attacks the Romans, that is disastrous for the Russian reputation. If Rome, however, takes back the Black Sea Coast after another war in 60 years, no-one will think any less of him for it, yet he and Russia did not agree upon a lease of the provinces for a NAP.
An interesting case is when treaties are agreed upon not as the result of armed conflict, but as the result of the threat of armed conflict. Essentially, the enforcing power is the threat of a lost war. Bavaria agreeing to cede a ruinous amount of German-cultured provinces in exchange for a NAP is equivalent to the example above. The alternative was more lost wars, with essentially the same outcome.
My point is that the vanquished party, having been forced to accede to demands, is not bound in spirit by those terms when the aggressor's force falls away. Another example is treaties signed on the basis of deceit: if one party tricked the other (essentially using force, albeit in a different way) and it is found out, the treaty will be considered far less valid than when parties engaged upon in out of their free will and fully aware of the consequences. So if you force a treaty onto someone, don't be surprised when you get screwed over on the letter of a treaty you had someone sign under duress. Catalunyan diplomats, at least, will not be surprised.
And Catalunyan diplomats will also try to arrange treaties that are beneficial to all signatories.