I agree with floppa21. And the SDI defense is key to winning Nuclear War. I just played Emperor on the largest map, so the AI would get big. I built one Nuc Missile -- that seemed to hold them off. But stacking many units in one city with SDI -- they would attack the same city over and over. I took no chances in expanding. I built a city within 3 squares of the next large enemy city, then captured it. The AI would respond by nuking their city I had just captured, but the SDI Defence held. Very fun!
Note: I had tried nuke retaliation, the global warming kicked in and destroyed the map, so I went back 20 turns to before the launch and played with SDI rather than many missiles. Also, Solar Plants seem to reduce pollution everywhere...