Here is a WSJ article about the effects of an EMP attack on the US. A lot of liberals say that the US doesn't need a ballistic missile defense because a terrorist could just slip in a nuclear weapon in a shipping container. But a single EMP device could do far more damage to America than multiple small nuclear devices. The EMP device has to be exploded high above the atmosphere, so a ballistic missile is needed to make it effective. Here is the article:

http://online.wsj.com/article/SB121824192073426161.html?mod=rss_opinion_main