US President George W Bush has welcomed seven former European communist countries to membership of the NATO military alliance.
The seven are Bulgaria, Estonia, Latvia, Lithuania, Romania, Slovakia and Slovenia.
The expansion is the biggest in the history of the North Atlantic Treaty Organisation and brings its membership to 26 countries.
NATO was founded in 1949 to protect western Europe from the Soviet Union. Although that danger has gone, President Bush said the alliance's mission is still the defence of its members against aggression.
Addressing delegates from the seven states on the White House lawn, Mr Bush spoke of Europe and America leading peaceful nations against the dangers of our time.
He said the new members understood the causes in Iraq and Afghanistan and were all participating in the war on terror.
Another ceremony to welcome the new members will be held in Brussels on Friday.