The US and Poland have signed a deal to locate part of the US's controversial missile defence system on Polish soil, BBC reported.
US Secretary of State Condoleezza Rice travelled to Warsaw for the ceremony, after 18 months of negotiations.
The deal has angered Russia, which has warned the base could become a target for a nuclear strike.
Washington says the system will protect the US and much of Europe against missile attacks from "rogue elements" in the Middle East such as Iran.
The agreement, which will see both US missiles and servicemen stationed on Polish soil, was signed by Ms Rice and Poland's Foreign Minister Radek Sikorski.
See map of US missile system
While Washington believes placing 10 interceptor missiles on a disused military base near Poland's Baltic Sea coast will protect much of Nato against possible long-range attacks, Warsaw sees threats much closer to home, says the BBC's Adam Easton in Warsaw.
That is why it demanded - in exchange for hosting the base - short-range Patriot missiles for its own air defences and a guarantee that the US will come to its assistance in the event of an attack, our correspondent adds.
The demands had delayed the deal's completion, but the conflict in Georgia gave the negotiations more impetus, says the BBC's Kim Ghattas, who is travelling with Ms Rice.
Both the US and Poland say the system is not aimed against Russia.
But the agreement has infuriated Moscow, our correspondent adds.
Russia's deputy chief of general staff, Gen Anatoly Nogovitsyn, said last week the plans for a missile base in Poland "could not go unpunished".
"It is a cause for regret that at a time when we are already in a difficult situation, the American side further exacerbates the situation in relations between the United States and Russia," he said.
Moscow has argued the project will upset the military balance in Europe and has warned it will be forced to redirect its missiles at Poland.
But Polish President Lech Kaczynski stressed the missile defence shield was purely a defensive system and not a threat.
"For that reason, no-one who has good intentions towards us and towards the Western world should be afraid of it," he said on Wednesday.
Before the conflict in Georgia there was a reasonable amount of popular opposition in Poland to the missile defence deal.
But new surveys show that for the first time a majority of Poles support it, with 65% expressing fear of Russia.