Critics of the Obama administration are suggesting that persistent Russian opposition to the plan is the explanation. Other theories include the enormous expense at a time when the American budget deficit is growing sharply and also continuing concerns that a missile defence system - first proposed by President Reagan in the early 1980s - is technologically too difficult. Some believe that a dirty nuclear bomb - smuggled into a major urban centre in a 'suitcase' - is much more likely and a missile defence system offers no protection against it.
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