Warsaw and Washington have signed a deal to deploy a US missile shield in Poland.
It has prompted deep anger and threats of retaliation from Moscow.
The United States says the shield will help it deal with new threats of the 21st century, saying there are long-range missile threats from countries like Iran and North Korea.
The signing comes amid heightened tensions between the United States and NATO with Moscow over Russia's conflict with Georgia.
The United States plans to base 10 interceptor missiles in Poland - plus a radar facility in the neighbouring Czech Republic - between 2011 and 2013.
It will complete a system already in place in the United States, Greenland and Britain.
Poland and the Czech Republic have been NATO members since 1999. Both are former Soviet Bloc nations.
The shield was endorsed by all 26 members of NATO earlier this year.
Russia has repeatedly protested that the missiles are a security threat on its border and has threatened to target the shield with its own missiles.