Military Defence vs. Wealth Cap

 Mortals are stupid. Imagine someone who isn't stupid. Whenever a defector deviates on a neutral or a cooperator, then all the cooperators turn against him. Shouldn't need any pre-existing pacts, the same way you shouldn't need special legal rules for removing wild bears from downtown. There isn't any reason to tolerate a defector, and if a cooperator wants help getting rid of him, don't think about it and help. I'll think about it anyway: the less the cooperator needs to spend to get rid of the defector, the more they'll have to trade with you, and the better off you'll be.

 However, mortals are stupid. They don't cooperate with cooperators. This triggers the wealth cap and ascerbity of revenge.
 If my society can't court any cooperators to help defend it, then that's my society's wealth cap. I can try some hack to get around the stupidity, but all such hacks will cost more than the gained defence. By definition, in this situation I can only try to rope in defectors. I have to deliberately associate with parasites. This will not be to my benefit. It will be a net loss. 

 Hence, we have a solution to the fact that Exit works poorly with military defence: so much the worse for military defence. Turns out defence pacts are deviant. Profane. Anti-property. Don't do them.
 If the government doesn't let you cancel your military subscription, it is a parasitic black government. What I need isn't soldiers for protection against foreigners, I need soldiers for protection against my government.
If we're going to have a constitution, then Exit implies the government must fund me a personal army, which I can trigger to harass and terrorize the government any time the government refuses to let me Exit in any way, shape, or form. 

 Imagine Trump could duel, to the death, the prosecutor and judge who were wronging him. Now imagine that he didn't have to, because it was his government-mandated right to have a small army specifically paid to do it for him. The only way for the prosecutor and judge to counter this clause would be to disown any right to affect Trump.
 That's what non-tyranny looks like. This is anti-Communism. It's not unfeasible. It's not even difficult. "You have the right to declare war on your government at any time and for any reason. You're not the traitor, the black government is the traitor." I mention it primarily to illustrate how far away from freedom any historical government has been. The second amendment is hopeless weaksauce, even if it worked as written; it's faking the right to punish, to vaccinate you against any meaningful right-to-punish clauses. All truly vile.

 Typically efficient defence is a poison pill. Rig the city with self-destruct explosives. Anyone trying to seize the city loses the city, along with any forces holding the city. Defeating invading armies isn't necessary, it's only necessary to inflict more damage than they can hope to gain in victory. Explosives are a cheap and simple way to achieve this capacity. If your city is worth $100 billion, then once you can inflict $200 billion in shockwaves and shrapnel, you're good to go. Costs like a couple million, basically a rounding error. Four million to rig it for self-destruct and to stockpile the deterrent at the same time.

 If they manage to take one of your cities in a surprise attack (again, impossible with the self-destruct stuff) then use the stockpiles to blow up two of theirs. I mean, the results are in: MAD works. Science replicates. MAD is not dangerous to citizens, it is only dangerous to the black government's ongoing lust for parasitism. 

Security is always affordable.

 That is, of course, unless the city is a defective city. Then blowing up the whole thing is a net gain for everyone else. Short-term costs for eternal-term profits. Do it almost regardless of the short-term costs. When genocide is the right answer.
 Though don't forget Revenge is Sour. If you can wipe out their city, you don't need to: threaten to wipe out their city if they don't shape up, and that will solve the problem 95% of the time.