Michael Gordon has a competent summary of debates surrounding a possible third missile defense interceptor site in eastern Europe.
The Missile Defense Agency, according to Gordon, argues that a site in Poland or the Czech Republic would “position the interceptors close to the projected flight path of Iranian missiles that would be aimed toward Europe or continue on a polar route to the United States.”
Too close, actually.
See, the earth is round. That means that Iran could use medium range ballistic missiles to fly under the radar in the UK and wipe out the interceptors. Ted Postol created a nifty diagram (above) that clearly explains why eastern Europe is a lousy choice for an interceptor site deployed against Iran:
I have attached for your information a diagram that shows the search coverage of the radar at Fylingdales with respect to both Poland and Iran. I have also plotted ballistic missile trajectories between Iran and Poland. The higher of the two trajectories uses a launch angle of 42° and the lower of the two shows a launch angle of 30°. The optimal trajectory (maximum range for a given burnout velocity) would have a loft angle of 37°.
This means that typical “minimum energy” trajectories from Iran to Poland would under fly the Fylingdales radar screen and never be seen. As such, a Czech or Polish missile defense site would need to have local radars and interceptors dedicated to self-defense.
… if Iran were truly involved in developing an ICBM, they would first have to develop a first stage rocket motor that would almost certainly be usable as a single stage rocket that could fly a 2000 plus pound payload from Iran to Poland or [the Czech Republic]. This therefore means that well before Iran succeeded in developing a postulated ICBM capability they would have the capability to attack or exhaust defense sites in either Poland or [the Czech Republic]. This could be done by launching nine or more of these much simpler shorter range missiles at the defense site. The operators of the defense would be forced to choose between expending all their available interceptors or letting a possible nuclear warhead detonate on Poland or [the Czech Republic]. Of course, none of the missiles would need to be armed with nuclear warheads as it would be impossible for the defenders to know whether or not a given rocket was carrying nuclear warheads.
Thus, the most simple of countermeasures, ones that do not even employ easily constructed decoys, could defeat such a defense site so imprudently close to Iran.