The United States switches on an $800 million missile shield in Romania on Thursday, a step it sees as vital to defend itself and Europe from so-called rogue states, but which the Kremlin says is aimed at countering Russia's nuclear capabilities in the event of war.
At the remote Deveselu air base in Romania, senior US and NATO officials will declare the ballistic missile defense site operational. It is capable of shooting down rockets from countries such as Iran that Washington says could one day reach major European cities.
"Iran continues to develop, test, and deploy a full range of ballistic missile capabilities and those capabilities are increasing in range and accuracy," said Frank Rose, deputy US assistant secretary of state for arms control.
"Iran's systems can reach into parts of Europe, including Romania," Rose said, before heading to the site to join US Deputy Defense Secretary Robert Work and NATO Secretary-General Jens Stoltenberg for a ribbon-cutting ceremony at 9am GMT.
When complete, the defensive umbrella will stretch from Greenland to the Azores. On Friday, the US will break ground on a final site in Poland that should be ready by the end of 2018, completing the shield first proposed almost a decade ago that also includes ships and radars across Europe.
It will be handed over to NATO control in July.
Russia is incensed at such of show of force by its Cold War rival in formerly communist-ruled eastern Europe. Moscow says the US-led alliance is trying to encircle it close to the strategically important Black Sea, home to a Russian naval fleet and where NATO is also considering increasing patrols.
"It is part of the military and political containment of Russia," Andrey Kelin, a senior Russian Foreign Ministry official, said on Thursday, the Interfax news agency reported.
"These decisions by NATO can only exacerbate an already difficult situation," he added, saying the move would hinder efforts to repair ties between Russia and the alliance.
The readying of the shield also comes as NATO prepares a new deterrent in Poland and the Baltics, following Russia's 2014 annexation of Crimea. In response, Russia is reinforcing its western and southern flanks with three new divisions.
The Kremlin says the shield's aim is to neutralize Moscow's nuclear arsenal long enough for the US to strike Russia in the event of war. Washington denies that.
"We are not meddling in anything that could be perceived as potentially destabilizing," said Douglas Lute, the US envoy to NATO.
However, Lute said NATO would press ahead with NATO's biggest modernization since the Cold War. "We are deploying at sea, on the ground and in the air across the eastern flanks of the alliance… to deter any aggressor," he said.
At a cost of billions of dollars, the missile defense umbrella relies on radars to detect a ballistic missile launch into space. Sensors then measure the rocket's trajectory and destroy it in space before it re-enters the earth's atmosphere. The interceptors can be fired from ships or ground sites.
While US and NATO officials are adamant that the shield is designed to counter threats from the Middle East and not Russia, they remained vague on whether the radars and interceptors could be reconfigured to defend against Russia in a conflict.
The US says Russia has ballistic missiles, in breach of a treaty that agreed the two powers must not develop and deploy missiles with a range of 500 km (310.69 miles) to 5,500 km. The US declared Russia in non-compliance of the treaty in July 2014.
The issue remains sensitive because the US does not want to give the impression it would be able to shoot down Russian ballistic missiles that were carrying nuclear warheads, which is what Russia fears.
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