An ICBM (inter-continental ballistic missile) has an unlimited range, and like the Tactical Nuke, has a devastating effect. They cannot be moved from the city that builds them but can strike any target on the map. Nuclear weapons, if used, have a tremendous negative effect upon world opinion.
The ICBM is the most powerful unit in the Civilization franchise. It has the ability to strike anywhere on the map, causing significant damage. When used over a city, it has a damage range of 12 squares. Inside the city, populations are reduced drastically and huge damage can be done to city improvements such as wonders or other buildings. Within 12 squares, the city is affected with pollution and radioactive waste. All the green pastures are destroyed, converting the area into desert. Another hit with an ICBM on the same city can send the city back to the prehistoric age, leaving it with a population count of 5 or less.
Other countries can use the ICBM against you and the amount of damage will be the same. However, the ICBM can be countered with the SDI. This gives you a 75% interception rate, destroying enemy ICBMs before they even reach your cities.
The use of the ICBM is likely to cause all the other civilizations to declare war on you and eventually use multiple ICBMs on you. However, if you managed to build the SDI defense system, you will be well-protected against missile attacks.
During war preparations, it is recommended for the player to build a large number of ICBMs. In case of war, you can direct a massive nuclear strike against any civilization, leaving their territory basically defenseless. Afterward, a few Mechanized Infantry or Marine units will be good enough to seize control.
The use of atomic bombs at Nagasaki and Hiroshima at the end of World War II changed the world's standards for measuring military power. Nuclear weapons can eradicate ground forces and armored divisions, and flatten cities with their awesome explosive power. In the years following World War II, arsenals of nuclear weapons were rapidly built by opposing nations, each fearing the capabilities of the other. This massive arms buildup has acted as a deterrent to full-scale war, since all the governments involved are aware of the consequences should such a war take place. The threat imposed by the huge nuclear arsenals of the world powers may one day be eradicated by the development of the Strategic Defense Initiative, or SDI, a system designed to destroy enemy missiles in flight before they can reach their targets.