Poland finally agreed on Thursday to host elements of US global anti-missile system on its territory after Washington improved the terms of the deal amid the Georgia crisis.
The preliminary deal was signed by deputy Polish Foreign Minister Andrzej Kremer and US chief negotiator John Rood. It still needs to be endorsed by the Polish parliament.
The signing comes after Prime Minister Donald Tusk had been holding out for enhanced military cooperation with the United States in return for consent to host 10 interceptor rockets at a base in northern Poland.
Washington says the interceptors and a radar in the Czech Republic would form part of a global "missile shield" protecting the United States and its allies from long range missiles that could in the future be fired by Iran or groups such as al-Qaeda.
"We have crossed the Rubicon," Tusk said just before the deal was signed.
"We have finally got understanding of our point of view that Poland , being a crucial partner in NATO and an important friend and ally of the United States, must also be safe."
Officials said the deal included a US declaration that it will aid Poland militarily in case of a threat from a third country and that it would establish a permanent US base on Polish soil in a symbolic gesture underlining the alliance.
"We are comfortable that we negotiated a strong agreement," Rood said. "It elevates our security relationship to a new level."
Moscow vehemently opposedIf everything goes to schedule, the interceptor base would be ready by around 2012, officials have said. The Czechs have already signed an agreement to host the radar although parliament there must yet ratify it.
Russia has vehemently opposed placing the shield installations in central Europe, saying they would threaten its security and upset the post-Cold War balance of power in Europe.
Moscow has threatened to take retaliatory steps against Poland and the Czech Republic, its former reluctant vassals who are now part of the European Union and NATO.
In the face of Russian opposition, Tusk had argued he could not agree to the shield unless the United States agreed to boost Warsaw's air defenses and enhance mutual military cooperation.
Russia's military action against Georgia strengthened the argument, Tusk said on Tuesday, ahead of the talks this week.
Polish Foreign Minister Radoslaw Sikorski played down the impact of the events in Georgia on the deal, apparently hoping to soften any criticism from Moscow.
In the first sign of Moscow's displeasure, Russian Foreign Minister Sergei Lavrov on Thursday cancelled a planned trip to Warsaw in September, Polish diplomats said.
The deal, if approved by parliaments in Prague and Warsaw, will be a rare success for President George W. Bush who has argued it is essential to contain the threat of a potentially nuclear-armed Iran.
Washington hopes the shield might persuade Iran to abandon its nuclear program, although Teheran says it wants to develop nuclear energy only to generate electricity and not to make nuclear weapons.