The United States is expected to formally announce its European missile-defense system operational on May 12, despite protests from Russia.
U.S. and NATO officials will gather in Romania to inaugurate a first-of-its-kind ground-based missile interceptor site, part of a larger shield Washington says is necessary to defend against Iranian ballistic missiles.
Russia fears that the shield could be used against Moscow and will upset the regional strategic balance.
The United States will also start construction on a second site in Poland on May 13 that is due to be completed in 2018.
Proposed in 2007, the shield comes as NATO prepares new deterrent capabilities in Poland and the Baltics following Russia's 2014 forceful annexation of Ukraine’s Crimea Peninsula.
In response, Russia has intensified military buildups in the region.
Based on reporting by Reuters and Stars and Stripes