To add to what I said above, you can give your vassals some of the benefits of SDI by stationing your units in their cities. When a nuke comes down on one of your units, your SDI will try to destroy it, and thus protect your vassal's cities. I'm not sure if this effect stacks with a vassal's own SDI. If it does stack, you might be able to do a lot of damage by vassalizing a psychopath, like Monty, and then gifting him tactical nuclear weapons and missile cruisers. When nuclear retalliation comes, an incoming ICBM will only have a 6.25% chance of hitting. Furthermore, you'll only have -1 diplomacy penalties for having Monty as a vassal, rather than the -20 "You nuked our friend!!!" penalties.
Note, of course, that after the first few nukes that hit, your unit will be dead, and the corresponding SDI effect will disappear.
Also note that you can't fire nukes at people you haven't declared war on. If you declare war on Isabella, and you want to nuke Madrid, there can't be any neutral or friendly units in the kill zone. This can be used to your advantage. Suppose you're in a situation where most of the world dislikes you, and you don't have a very powerful military. You can't afford to declare war, both because of the diplomacy penalties, and the lack of overwhelming military power. Suppose your ally, Saladin, is at war with Isabella, and Isabella has lots of nukes. You can station a unit in each of Saladin's cities and Isabella will be unable to nuke him unless she declares war on you. If you want to be an . .. .. .. .. .. .. ., you can gift tactical nuclear weapons to Saladin, and allow him to nuke Isabella without fear of retalliation.
