[Xietor]

The Forge is another Global that could end the NAP. The Forge is sufficiently powerful to be in a class that the caster is saying, in effect, "I win" stop me if you can.

Arcane Nexus is clearly in that class, and the Forge likely is as well.

Any global that causes me direct damage also violates the NAP. Wrath of Gods. One of my guys get struck by lightning, I could(but may choose not to) consider the nap terminated.

A smart player who does not want to alienate a powerful ally would inform him in advance of his intent to cast the Forge, Wrath of Gods etc., and get his blessing. There may well be a price for that blessing, such as forging x number of items a turn.

I also never forget a treaty breaker. And my advice is to make sure you really think breaking the nap is worth the gain.
Good players are likely not going to be taken by surprise. And they are unlikely to leave key assets undefended even if they have treaties.

But if someone breaks an nap with me, that carries over to the next game and so on 4ever. It never goes away. They are always unworthy of trust. And that means you are limiting your diplomatic options by breaking naps as well. It can be a heavy price to pay for a small gain.