I want to check if the geopolitical system of my world is actually stable. Here's the background on the HAMMER protocol, the treaty that unwound Mutually Assured Destruction:
MAD gets tested, but doesn't happen:
All nations got nuclear ICBMs about 350 years ago. Mutually Assured Destruction reigned absolute for a while, but then about 70 years after nuclear armament, MAD was tested when a country called Vaaison launched a strike on the Gensuran Republic. Surprisingly the world was not destroyed. The countermeasures available were known to be insufficient, but they could hold off an attack for about fifteen minutes before the first missile got through the grid. In this critical time, the allies of Vaaison decided they would rather lose an ally than destroy the world, and immediately broadcast on all channels that they did not intend to fire, and disowned Vaaison. Satellite and radar tech was good enough to observe they were making good on their intent, and not firing. So Vaaison lost all its allies and was subsequently hit by retaliatory strikes from everyone. Because of the overwhelming retaliation, only a few Vaaison ICBMS got through to their target cities before Vaaison's defenses critically failed, and the whole country became a radioactive crater.
HAMMER replaces MAD
This incident lead to the adoption of the HAMMER protocol by international treaty. HAMMER got its name from the phrase "The nail that sticks up is hammered down" which is now often said as "The state that nukes first is HAMMER-ed down). Under the HAMMER protocol, initiating nuclear war immediately nullifies any alliances or mutual defense pacts your country has, leaving you at war against the world. Being the first to fire a nuke makes you the nail, and everybody else hammers. All other countries will work together to annihilate you. Getting caught trying to form an alliance that subverts HAMMER, or making demands with the threat of nuclear war also violate international treaty, but aren't automatic triggers of the full HAMMER protocol. The HAMMER protocol has teeth because as was shown by Vaaison, the destruction is not mutual. The destruction is assured for the instigator, but survivable by the others.
Conventional war returns:
HAMMER ended up being MAD turned upside-down, because the protocol is only activated by nukes, not conventional warfare. About 250 years ago, two nations got in a territorial skirmish over mineral resources, and what started as some fighter jets playing chicken and ground troops harassing each other escalated into war with conventional weapons. HAMMER actually let war happen without fear of MAD. Neither side believed the other would launch nukes first. How believable are threats that you'll launch a nuke over this coltan mine if it would mean total annihilation of you and your people?
The new equilibrium:
Ever since wars have been fought without nukes. They are always lurking in the silos, but nobody uses them. When a nation is losing badly, a leader with launch codes might try to posture and look crazy (or actually be crazy) enough to choose suicidal nuclear rage-quit over surrender, and that threat might lead to an armistice slightly more favorable than the loser could have otherwise negotiated, but other than that, the days of MAD are gone. More than one leader has threatened nuclear action only to be immediately assassinated by his or her own cabinet, who, faced with face-melting HAMMER annihilation, remembered that defending their values to the death had been just a campaign slogan.
Could nations go to war under this system without triggering nuclear cascade? Is HAMMER a stable geopolitical state like MAD? What would be the points of failure in the system? Is there a series of moves a key player could make to either seize nuclear dominance or send the world back to MAD?
Extra background on my specific world:
The geopolitics of the world are less like ours in the modern day, and more like feudal city-states. Like territory being taken and taken back in early Europe, people weren't horrified and screaming for all-out war if a part of their outer territory was taken, like Americans would be if one State got occupied. The politics between most nations is like that between early England and France. Always bickering and fighting over things and moving the borders of occupation and raiding each other, but not getting the whole continent swept up in a total war. The people of my world are loyal to the city-part of their city-state, but the other parts of their territory they have learned to be less attached to. Sometimes you own that lesser city or those oil fields, sometimes you don't. You'll take it back next year. And the people kinda like fighting and don't form larger alliances very well. So when nukes were developed and MAD started, tensions were very high because everyone wanted to go back to fighting, it was simpler than politics.