US President George W Bush has welcomed seven former communist eastern European countries as new members of Nato, during a ceremony in Washington.
Mr Bush said the new members bring "moral clarity" to Nato
He said Bulgaria, Estonia, Lithuania, Latvia, Romania, Slovakia and Slovenia joined as "full and equal partners".
It is the biggest expansion in the history of Nato, created in 1949 to defend western Europe against the Soviet Union.
A second ceremony will be held at Nato's HQ in Brussels on Friday.
With the admission of the seven new countries, Nato now has 26 members.
Mr Bush welcomed to the White House the prime ministers of all seven new members as well as Nato Secretary General Jaap de Hoop Scheffer.
"As witness to some of the great crimes of
the last century, our new members bring moral clarity to
the purposes of our alliance," he said.
"They understand our cause in Afghanistan and in Iraq... because tyranny for them is still a fresh
"And so now as members of Nato they are stepping forward to secure the lives and freedoms of others," he added.
Earlier, US Secretary of State Colin Powell presided over a formal ceremony at the State Department.
He said the new members would form the vanguard of Nato's determination to support the "yearning for freedom" of people around the world.
Romania and Bulgaria have been two of the most enthusiastic pending Nato members, with the majority of the population in both countries supporting admittance to the alliance.
In Bulgaria, there will be a national holiday on Friday when the Brussels ceremony takes place.
Nato plans to expand operations in countries such as Afghanistan
Albania, Croatia and Macedonia are also seeking to join the alliance and Mr Bush will also meet the prime ministers of those states on Monday.
Since the end of the Cold War, Nato's frontiers have moved steadily eastwards; first taking in Poland, Hungary and the Czech Republic, now extending to Romania and Bulgaria's Black Sea coast and - with the three Baltic republics - northwards almost to Finland.
The Baltic republics - Lithuania, Estonia and Latvia - used to be part of the Soviet Union and Russia has expressed irritation over their inclusion, fearing threats to its security and US interference in the region.
Nato has agreed to include the Baltic states under its air defence shield and is enforcing it by stationing four F-16 fighter planes in Lithuania.
New US bases
BBC central Europe correspondent Nick Thorpe says that despite its resistance to the inclusion of the Baltic states, Russia has done little more than grumble and its complaining has not caused significant debate.
BBC defence correspondent Jonathan Marcus says that alliance membership is a rite of passage, providing the new member countries with a confirmation of their own transformation into democratic, market-oriented states.
Nato itself is changing, taking on new missions in Afghanistan and possibly in Iraq and is looking towards its southern flank with North Africa amidst growing concerns about terrorism, he adds.
And Washington is already eyeing the territory of some of the new Nato members as potential locations for military bases from which to project US power into the greater Middle East, our correspondent adds.