Editing Strategic Defense Initiative (section)

==Countermeasures==
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[[File:Ground-Space based hybrid laser weapon concept art.jpg|thumb|An artist's concept of a ground / space-based hybrid laser weapon, 1984]]
In war-fighting, [[countermeasure]]s encompass multiple meanings:

* Immediate tactical action to reduce vulnerability, such as [[Chaff (radar countermeasure)|chaff]], [[decoy]]s, and maneuvering
* Counter strategies that exploit a weakness of an opposing system, such as adding more warheads that are less expensive than the interceptors fired against them.
* Defense suppression - that is, attacking elements of the defensive system.

Countermeasures have long been a key part of warfighting strategy; however, with SDI they attained a special prominence due to the system cost, scenario of a massive sophisticated attack, strategic consequences of a less-than-perfect defense, basing many proposed weapons systems in space, and political debate.

Whereas the [[United States national missile defense]] system targets a relatively limited and unsophisticated attack, SDI planned for a massive attack by a sophisticated opponent. This raised significant issues about economic and technical costs associated with defending against [[anti-ballistic missile defense countermeasures]] used by the attacking side.

For example, if it had been much cheaper to add attacking warheads than to add defenses, an attacker of similar economic power could have simply outproduced the defender. The "cost effective at the margin" requirement was first formulated by [[Paul Nitze]] in November 1985.<ref>{{cite web|first=Marilyn |last=Berger|url=http://www.sais-jhu.edu/pubaffairs/SAISarticles04/Nitze_NYT_102004.pdf |title=Paul Nitze, Cold War Arms Expert, Dies at 97 |archive-url=https://web.archive.org/web/20060910113725/http://www.sais-jhu.edu/pubaffairs/SAISarticles04/Nitze_NYT_102004.pdf |archive-date=September 10, 2006 |publisher=-[[New York Times]]|date=October 20, 2004}}</ref>

In addition, SDI envisioned many space-based systems in fixed orbits, ground-based sensors, command, control and communications facilities, etc. In theory, an advanced opponent could have targeted those, in turn requiring self-defense capability or increased numbers to compensate for attrition.

A sophisticated attacker having the technology to use decoys, shielding, maneuvering warheads, defense suppression, or other countermeasures would have multiplied the difficulty and cost of intercepting the real warheads. SDI design and operational planning had to factor in these countermeasures and the associated cost.