President George W Bush has welcomed Lithuania into Nato, saying its admission will "lift and invigorate" the alliance.
Thousands of people turned out in the capital, Vilnius, to hear Mr Bush - the first American president to visit the country.
In his speech, he said the decision to admit Lithuania and six other former communist states would inject new life into the stale structure of the organisation.
"The Nato alliance will be stronger with Lithuania's presence," said Mr Bush.
"Not only will you help militarily but, as importantly, your presence will help lift and invigorate the spirit of the European North-Atlantic alliance."
The Lithuanian President Valdas Adamkus then presented Mr Bush with the Cross of the
Order of Vytautas the Great, Lithuania's highest honour.
"It is a great moment for us," he said.
In return, Mr Bush presented Mr Adamkus - who lived for many years in the US after fleeing the Red Army - with a basketball signed by legendary player Michael Jordan.
Mr Bush has become the first American president to visit Lithuania, which along with its Baltic neighbours Latvia and Estonia won independence in 1991 after half a century of Soviet occupation.
Apart from the Baltic states, Nato on Thursday formally invited Bulgaria, Romania, Slovakia and Slovenia to join the alliance.
Mr Bush is scheduled to travel from Lithuania to Romania for the last leg of his whirlwind European tour.
He is to address thousands of people in central Bucharest's Revolution Square - the place that symbolises the collapse of communism and the overthrow of President Nicolae Ceausescu 13 years ago.
Looking west
The BBC's Nicholas Walton, in Vilnius, says Mr Bush's visit underlines the feeling in the Baltic republics that Russia will no longer carry such influence in the region.
The Latvian President, Vaira Vike-Freiberga, said the visit would be "a celebration of the Baltics, the United States and, of course, Nato".
Soviet troops clashed with demonstrators in Vilnius in 1991
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Earlier in the week Mr Bush told Lithuania's LNK television that "to have these countries allied with the United States and other countries is important to our soul".
Almost one million Americans claim Lithuanian ancestry, and the United States never recognised Lithuania as part of Soviet territory during the Cold War.
Mr Bush met Russian President Vladimir Putin in St Petersburg on Friday - and sought to reassure him that Nato's moves to expand to the east were in Moscow's "best interests".
Mr Putin voiced scepticism, saying: "We do not believe that this [expansion] has been necessitated by the existing facts, but we take note of the position taken by the US president".
Mr Bush stressed that the view at Nato was that Russia was a "friend" and he called for continued co-operation in the US campaign against terrorism.
Mr Putin did not attend the Prague summit, in order to avoid appearing to welcome Nato's invitation to the three Baltic states.