"No one has high confidence that the U.S. missile defence system would have anything approaching 100 per cent accuracy in shooting down incoming missiles."

Correct me if I'm wrong but to date the only successful anti-missile tests have come when the defenders have plenty of warning and the missile's trajectory is known in advance.  So, in reality, nobody has the slightest idea whether these things will work in real world conditions.  That's a lot less than "100 per cent accuracy."