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The SDI Defense (Civ2) SDI Defense[note 1] is a defensive city improvement in Civilization II, available with The Laser advance. It protects everything within a three-tile radius of its host city against nuclear weapons, and doubles garrisoned units' defense against conventional missiles.


The SDI Defense is the only countermeasure against a nuclear attack. Foreign missiles that attempt to attack targets within the SDI's range are immediately intercepted and destroyed,[note 2] with both players notified by an advisory popup. SDI will not intercept a player's own missiles, nor does it prevent blast damage from attacks against the fourth tile outside its perimeter.

SDI is a priority for the player's most populous and prosperous cities: each nuclear strike halves citizen count, so built-up metropoli suffer greater devastation than small towns. Total destruction of the garrison also leaves a city vulnerable to capture by Paratroopers, a common tactic by computer opponents. Due to SDI's high Shield (Civ2) Production cost, it may be necessary to buy the improvement outright to protect newly-captured cities from retaliatory strikes: the AI does not discriminate between foreign cities and its former possessions.

SDI doubles the city garrison's defense against Cruise Missiles. This bonus compounds with the SAM Missile Battery: a city with both SAM and SDI gains a fourfold defense.

SDI cannot be targeted for sabotage by espionage units.[note 3] Cities with SDI remain vulnerable to "suitcase nukes", however these pose the risk of triggering global condemnation even if the player is at war with the target.

Civilopedia entry[]

Historically, research in the area of warfare has seen the development of powerful new weapons closely followed by the development of defenses to counteract the weapons' destructive power. The development of nuclear weapons seemed to be the exception to this rule: for the first time, mankind had created a weapon to which there was apparently no effective defense. SDI (Strategic Defense Initiative), however, could be the answer to this problem. Using laser-equipped orbital satellites or ground-launched ABMs (Anti-Ballistic Missiles) to intercept and destroy enemy nuclear missiles in mid-air, the SDI Defense offers the first hope of negating the threat posed by nuclear missiles.


The SDI Defense is defined on Line 18 of the @IMPROVE section of Rules.txt. Popup messages for successful interception, and thwarted sabotage, are defined in Game.txt at @SDI and @SABOTAGENO, respectively.


  1. "SDI" is the acronym for the Strategic Defense Initiative; hence, "SDI Defense" is redundant.
  2. Intercepted missiles are not logged as casualties by the Defense Minister.
  3. Anecdotal reports of SDI sabotage may indicate a bug.

See also[]

City improvements in Civilization II [edit]
Efficiency Corruption
Growth Food
Industry Production
Science Research
Related pages
1 Only featured in Test of Time.