The walls of Troy held all enemies off for many years, except for earthquakes, and according to legend, held the Greeks back for almost a decade.

The Germans had to go around the Maginot Line, and only one of its fortresses fell before the French surrender. Had the line been extended further than it was (admittedly, at ruinous expense), it might have been a different story, although IMO the air war would still have been a deciding factor.

Only idiots believe that a missile defense could be infallible. But as others have pointed out, if just one warhead is prevented from hitting a city, the system will have paid for itself. It's estimated that the financial cost to just New York State of the 9/11 destruction approaches $100 billion. 9/11 also proved that deterrence and fear of retaliation isn't really a factor for some people.

I do know that the failure rate of a system that is never built is 100 percent.