U.S. Secretary of State Condoleezza Rice and Polish Foreign Minister Radek Sikorski signed a deal Wednesday that will put an American missile defense base in Poland, a move that has angered a resurgent Russia.
The formal signing comes six days after the two countries agreed to a deal that will see 10 U.S. interceptor missiles placed just 115 miles (180 kilometers) from Russia's westernmost frontier.
"The negotiations were very tough but friendly," Prime Minister Donald Tusk said to Rice in English, after the signing.
"We have achieved our main goals, which means that our country and the United States will be more secure," he said.
Rice underlined that point, saying: "It is an agreement which will help us to respond to the threats of the 21st century."
The deal has sparked threats from Russia that Poland is making itself vulnerable to attack - even a nuclear one. Along with Russia's rhetoric, the agreement has further strained Moscow's ties with the West in the wake of its fighting with U.S. ally Georgia.
Before signing the deal, Rice emphasized that the site is not intended as a threat.
"This is a system that is defensive and is not aimed at anyone," she said. "This is an agreement that will establish a missile defense site ... that will help us to deal with the new threats of the 21st century of a long-range missile threats from countries like Iran or from North Korea."
President Lech Kaczynski expressed "great satisfaction" at the outcome after months of negotiations.
"Both sides have achieved their goals. This is a great success for Poland," Kaczynski said, standing alongside Rice in the Presidential Palace gardens.
As well as clearing the way for the U.S. missile defense shield to take shape, the deal is expected to further deepen Washington's military partnership with Poland, an ally in Iraq and Afghanistan.
The U.S. firmly rejects Moscow's insistence that the system is a threat to Russia. Yet Poles - the majority of who initially rejected the idea of hosting the site - have come to see it as offering a form of protection beyond NATO in light of Russia's recent military action in Georgia.
Warsaw and Washington spent a year and a half negotiating, and talks recently had snagged on Poland's demands that the U.S. bolster Polish security with Patriot missiles in exchange for hosting the missile defense base.
Washington agreed to do so last week as Poland invoked the Georgia conflict to strengthen its case.
The Patriots are meant to protect Poland from short-range missiles from neighbors - such as Russia.
Kaczynski also has stressed that the missile defense shield is purely defensive.
"For that reason, no one who has good intentions toward us and toward the Western world should be afraid of it," he said in a televised nationwide address on Tuesday.
Poles have been shaken by Russian threats against their nation as punishment for accepting the U.S. site.
A day after Warsaw and Washington reached agreement on the deal last week, a leading Russian general made his country's strongest warning to date against the system.
"Poland, by deploying (the system) is exposing itself to a strike - 100 percent," Gen. Anatoly Nogovitsyn was reported as saying Friday by the Interfax news agency.
In the interview, Nogovitsyn pointed out that Russian military doctrine permitted the use of nuclear weapons in such a situation, Interfax reported.
The Russian Baltic Sea exclave of Kaliningrad is wedged between Poland and fellow European Union and NATO member Lithuania. Russia keeps weapons and troops stationed in Kaliningrad.
Poland got support from NATO on Tuesday at an emergency meeting in Brussels, with NATO Secretary General Jaap de Hoop Scheffer denouncing the Russian threats with unusually strong language.
"It is pathetic rhetoric," he told reporters. "It is unhelpful and it leads nowhere."
The U.S. already has reached an agreement with the government in Prague to place the second component of the missile defense shield - a radar tracking system - in the Czech Republic, Poland's southwestern neighbor and another ex-communist country.
That deal still needs approval from the parliaments of both the Czech Republic and Poland.
No date has been set for the Polish parliament to consider the agreement, but it should face no difficulties in Warsaw, where it enjoys the support of the largest opposition party as well as the government.