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In developing their network of defences, the Americans have designed or are in the process of developing systems that potentially would be capable of destroying an incoming ballistic missile at the three phases of its journey: the boost phase, when it is first launched, mid-course and terminal.

The boost stage is the trickiest, and work is still under way to find the ideal defence, but the Americans have been developing an airborne laser fitted to a Boeing 747, and trials so far have proved successful. The first flight test of the laser is scheduled to take place next year. Work is also under way to develop a high-acceleration mobile interceptor that could be deployed close to Iran or other threat country. But both these programmes are up to ten years away from having an operational capability.

For the mid-course phase, the Americans are already in business. Interceptors based in Alaska and California have been operational since 2004 to protect the US from the perceived threat from North Korea. The interceptors, like the ones that will be located in Poland, have no warhead. They are “hit-to-kill” systems, guided by sensors to smash into an incoming missile at a closing speed of 15,000 miles an hour. Even a glancing blow would be enough to destroy the enemy missile.

There are currently 21 interceptors in silos in Alaska and three in California. A total of 30 interceptors will be in place by the end of the year. As part of this system, the RAF early warning radar station at Fylingdales in North Yorkshire has been upgraded, providing a missile-tracking capability to counter the threat from Iran. There is also a radar facility in Japan. The interceptors in Alaska and California, the largest of which is 60ft long, are also able to address the Iranian threat.

Additionally, the US Navy has 15 Aegis class warships equipped with Standard interceptors (SM3s) which are also capable of hitting enemy ballistic missiles in the mid-course phase. By the end of the year there are planned to be 18 Aegis ships with a combined total of 34 SM3 interceptors.

For the terminal phase, the Americans can use the Aegis warships, but also their well-tested Patriot weapon system — the latest model is called PAC3. In addition, they are developing the Terminal High Altitude Air Defence (THAAD) system, a mobile weapon capable of intercepting a ballistic missile inside or outside the Earth’s atmosphere.

How successful have the Americans been with their interceptor trials?

According to the US Missile Defence Agency, 35 out of 43 hit-to-kill intercepts have been successful, including five out of five THAAD tests, 13 out of 15 Aegis trials, six of the nine tests with the long-range interceptors of the kind in Alaska and California, and 11 out of 14 Patriot trials.

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