In your example there is nothing to be gained by not complying.

Failing to comply with what is in reality arms control is a widespread occurence. It's notable the only historical way to handle this (Schelling covered it at length) is tolerance of mutual espionage on a level that would simply not be tolerable to any of the groups involved.

I should add one of Schelling's other pet subjects was "time to weapon being viable". i.e. the distinction between a nuclear power and non-nuclear power isn't quite right, as there's a huge class of countries that could become so fairly fast if they were so inclined, and that this situation is somewhat inevitable. It is this that leads to efforts to denuclearize, even on a civilian level, "problematic" countries. You want to stop frontier AI and you'll have to be rolling back a lot of existing deployed technology.

Great comment. I haven't read Schelling but will check that out. I think you're right that international cooperation on the topic is likely impossible at present, for the reasons you stated. I'm not sure what it would take to shift the Overton window, but it's probably not pretty.