The closing of the deal between the United States and Poland for ten interceptor missiles to be based on Polish territory completes the final component for a missile-defence system that has grown in complexity and sophistication.

It is wrong to describe the American missile system as a “shield” because that implies total invulnerability from hostile attack.

However, in the past few years the US has developed and, in some cases, brought into service, an extraordinary array of equipment that should meet Washington’s objective — to protect the territory of the United States, its troops serving in the Middle East and its allies in Nato from a limited rogue nuclear attack from North Korea and Iran.

The Axis of Evil declared by President George W Bush is now confined to the potential ballistic missile threat emanating from these two countries, particularly Iran now that North Korea has at least begun to talk and act more sensibly about its nuclear weapons programme.

Iran is key to the whole anti-missile structure devised by the US, and now helped on its way with the agreement by the Polish Government to host ten interceptors at a base in northern Poland. The Czech Republic has already given the go-ahead for the other essential ingredient, a radar system capable of detecting and tracking the launch of an enemy ballistic missile. Both the radar and the interceptors should be in place by 2011-13.

Why Poland and the Czech Republic?

The US wanted to extend coverage of the missile system to European allies, as well as American troops serving in the Middle East. To guarantee sufficient protection, it was necessary to deploy part of the system in Europe. US experts looking at the potential trajectories of a missile launch from Iran against either America or Europe concluded that Poland and the Czech Republic were best placed geographically to take any initial action. Range was also important. The interceptors to be deployed in Poland will have a range of 3,000 kilometres (1,800 miles).

As the Russians have been told on numerous occasions, ten interceptors based in Poland would not be capable of knocking out the hundreds of missiles, fitted with thousands of warheads that are in the Russian nuclear inventory. However, although the Cold War is over and Russia is not considered likely ever to launch a pre-emptive nuclear strike against the West, Moscow is still apparently worried that the US missile defence system so close to its borders will somehow undermine the credibility of its nuclear force, and, thus, call into question its independent deterrent.

What is the threat from Iran?

American intelligence reports have concluded that Iran is trying to develop an extended-range version of its Shahb-3 missile as well as a new 2,000-kilometre ballistic weapon, the Ashura. These missiles would have a range capable of going well beyond the stated aim of President Ahmadinejad, the Iranian leader, of “wiping out” Israel. The Americans are also concerned that as Iran develops long-range ballistic missiles, possibly with nuclear warheads at some stage in the future, Iranian scientists and engineers will also succeed in including multiple decoy systems to counter the US anti-missile network.

The US believes that Iran will have a long-range ballistic missile capable of reaching America by 2015, which is why Washington has been so keen to sign agreements with Poland and the Czech Republic to host, respectively, the interceptors and radar facility in time for that deadline. To try to appease Russia’s concerns and to convince Moscow that the interceptors are needed only to counter the threat from Iran and other like-minded rogue states, Washington has said that it is prepared not to switch on the Polish and Czech systems until 2015.

What systems are already in place in the US network?

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