Let's say some country intends to introduce a new aviation policy that conflicts with ICAO regulations, like only aircraft certified and approved by our government can fly in our airspace. Can they do that unilaterally? Has it ever happened?
For instance, Russia may declare a country as enemy country and it may use its equipment (e.g reconnaissance aircraft) to spy on this enemy country. In this case enemy country may start defining its own policies and make sure that defense or civil aviation equipment/software are used in compliance with these new policies.