The Aegis Ashore site includes a powerful radar, missile interceptors and communications equipment.
The system has been handed over to military commanders, who are expected to declare it operational, or ready for initial combat use, in early 2016 as they integrate the site with NATO's broader ballistic missile defense system.
Russia threatens 'consequences' if US deploys missiles in Europe
It coincides with deliberations by US officials about how to respond to an Iranian ballistic missile launch on Oct. 10 that violated UN Security Council resolutions.
US ambassador Hans Klemm, speaking at a joint ceremony with Romanian Foreign Minister Lazar Comanescu, said the shield
was designed to thwarth potential threats coming from outside Europe.
Both Klemm and Comanescu stressed it was not a measure against Russia.
Iran missile launch violated UN resolution: UN experts' report
"It is not, I repeat, not, directed at Russia, nor does it have the capability to threaten Russia," Klemm said. "We have explained this to Russia on numerous occasions."
The Iranian test has intensified concerns among US lawmakers about an international nuclear deal in which Iran agreed to curb its nuclear program in exchange for relief from economic sanctions.
A team of sanctions monitors found on Tuesday that Iran violated a UN Security Council resolution by test-firing a missile capable of delivering a nuclear warhead.
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