This trend of warfare where you just blast away at someone with ICBMs until they capitulate is a new thing to me, and I don't like the calculatedness of it. There should be some unpredictable element to any form of warfare, even if it's not the weapons themselves.
So I'm just going to throw a couple of ideas to you guys and let you kick them around for a bit.
1 new building: Missile Silo. Underground building, construction cost of 2-3 steel. 1 intrinsic to your battle mine, and you could build 2-3 on your land. I don't think the number of silos available to you should be tech-dependent, as that would give a combat advantage (if a costly one) to older players, which is not how BM works. A missile silo performs the same function for ICBMs as cannons do for short-range munitions. You'll always be able to fire at least one, but you could fire as many as 3 or 4 at once.
1 new policy setting: Salvo Size. A number from 1-4 (or 1-3). Determines how many nukes you will fire (assuming you have them, and enough silos to fire them) if you initiate distant warfare or have distant warfare initiated upon you. However, as with Firepower, your Salvo Size would be available to anyone who has their Palantir with you, and is holding their own Palantir. Nothing's perfect.
The problem with the second idea is that most people don't have a stockpile of 8+ ICBMs at a time, thus you would just be perpetuating the cycle of calculated ICBM warfare for those who stockpile (Which is anyone who is planning a rampage anyway).
The first idea is great - make the building require 1 EUR as ammunition to fire the free ICBM on defense.
I don't think there's anything wrong with the system there is at the moment. It makes sure people keep a few ICBMs as defense: you know that if you have 2 ICBMs, no-one will attack you unless they have 3 or more, and they're much more likely to go for an undefended person anyway. The planning can work out to the advantage of the defender, not just the attacker.