The US has inaugurated the new Aegis Ashore Phase II radar site and missile battery in a ceremony held in Bucharest, Romania.
The deployment is expected to strengthen security in the Euro-Atlantic area.
US Army Corps of Engineers (USACE), Europe District and Missile Defense Agency, US Navy and Romanian officials have cooperated to complete the $170m missile defence complex.
The Aegis Ashore Missile Defense System (AAMDS) at Devesulu in Romania constitutes an intrinsic part in Phase II of the European Phased Adaptive Approach (EPAA).
US 6th fleet commander vice admiral James Foggo said: "Missile Defense Agency director vice admiral Syring and I conducted a thorough walk through of the Aegis Ashore site in Deveselu yesterday.
"Now that Commander, US Naval Forces Europe-Commander, US Naval Forces Africa (CNE-CNA)/US 6th Fleet has ownership of Aegis Ashore-Romania, we will begin integration into the existing Nato Ballistic Missile Defense (BMD) architecture."
The measure is seen as a defensive strategy to shield the US and its allies against ballistic missiles from Iran and the Middle East.
The missile site is spread across an old 430-acre Romanian air base, refurbished as the naval support facility at Deveselu in October 2014.
The site will be guarded by sailors rotating through for six months at a time. It will be equipped with an Aegis SPY-1 radar system, and vertical-launch missile system armed with long-range anti-air missiles.
The land-based facility is modelled on a system akin to that used on the US Navy Aegis-capable guided-missile destroyers and cruisers. It can track and destroy airborne ballistic missiles.
The three construction contractors involved with the project are SC Glacial PROD SRL of Romania, the US firm KBR, and Zafer Construction.
The Phase II precedes Phase I, which was completed in fiscal year 2011.
It involved the deployment of current and proven missile defence systems, including one deployed BMD capable ship, the SM-3 Block IA interceptor, and a forward-based Army Navy / Transportable Radar Surveillance system (AN/TPY-2) in Turkey.
Image: US Ambassador to Romania Hans Klemm speaking during a ceremony in Bucharest. Photo: courtesy of Lucian Crusoveanu, State Department.