US president Barack Obama says the new missile-defense system in Europe will provide American forces and their allies with stronger, smarter, and swifter defense. But how does the system work, and what does it comprise?

The shield relies on radar to detect a ballistic missile launch into space.

Tracking sensors then measure the rocket's trajectory and intercept and destroy it, before it re-enters the earth's atmosphere.

The interceptors can be fired from ships or ground sites.

The Romanian installation is the first land-based defensive missile launch system in Europe.

It now joins other elements of the NATO defensive shield, including a command-and-control center at the Ramstein Air Base in Germany.

The US planned to build its European missile defense system in three different phases.

Starting in 2011 with Phase 1, it deployed a missile defense radar in Turkey and began the sustained deployment of the Aegis Ballistic Missile Defense, or BMD-capable ships in the Mediterranean. In September last year it deployed the fourth and final U.S. Aegis BMD-capable ship at the naval facility in Rota, Spain.

The US completed the deployment of the Aegis Ashore site in Romania in December of last year, which works alongside BMD-capable ships in the Mediterranean. 

It will also start construction on a second site in Poland that is due to be ready in 2018, giving NATO a permanent, round-the-clock missile shield, in addition to radars and ships already in the Mediterranean.