The entry of Estonia, Latvia, Lithuania, Bulgaria, Romania, Slovakia and Slovenia in a triumphant ceremony on Monday increased the members in the US-dominated alliance to 26.
In an immediate reflection of the shift eastward of an alliance forged to fight the Cold War, NATO fighter jets headed to the Baltics, Secretary-General Jaap de Hoop Scheffer said.
"Welcome to the greatest and most successful alliance in history," US Secretary of State Colin Powell told the nations' prime ministers, who formally handed over their accession documents.
Despite fears the enlargement could hamper timely deployments because NATO needs consensus on military action, the top US diplomat also said he supported the ambitions of Albania, Croatia and Macedonia to one day join the alliance.
The new members rejoiced at joining an organisation which ensures military protection to its members.
"Today, it is really fantastic day for Slovakia. I consider this a very big success," Foreign Minister Eduard Kukan told Reuters.
Forty per cent of NATO will now be former communist states.
Russia has bitterly criticised the enlargement, especially for nations that formed part of the Soviet Union until 1991. On accession day, a Russian parliamentary deputy dismissed the Washington ceremony to formally receive the seven allies' acceptance documents as a "show".
"It's understandable that the Americans are putting on a show today," Konstantin Kosachev, representative of a Russian parliamentary committee on international affairs, told journalists.
He said a NATO plan to patrol the airspace of the three Baltic states was an "unfriendly" move. Estonia and Latvia border Russia, while Lithuania has a frontier with Moscow's Kaliningrad enclave.
"Accepting the Baltic countries and arranging a guarantee for their security, many within NATO apparently proceeded from a previous idea that there could be a war in Europe"
Russian Foreign Ministry
"It can not be ruled out that Russia ought to look at the possibility of taking corresponding measures."
Russian Foreign Ministry spokesman Alexander Yakovenko said in a statement: "Accepting the Baltic countries and arranging a guarantee for their security, many within NATO apparently proceeded from a previous idea that there could be a war in Europe.
"The main thing that could improve the state of European security is a fundamental change in the very nature of NATO, a consistent implementation of the agreement of the new quality of the relationship between Russia and NATO, including a joint fight against new and real threats and challenges."
But the expansion could hinder NATO's ability to respond quickly to perceived threats because of its consensus decision-making.
Last year, a month-long tussle over whether to bolster Turkey's defenses ahead of the unpopular Iraq war is an example of how action can be blocked by a few nations willing to defy the United States.
Last year, a month-long tussle over whether to bolster Turkey's defenses ahead of the unpopular Iraq war is an example of how action can be blocked by a few nations willing to defy the United States
Since then tempers have cooled. NATO has taken command of the multinational peacekeeping mission in Afghanistan - its first operation outside Europe or North America.
But the rifts over the US-led Iraq invasion, which did not win UN approval, have not been fully healed.
NATO's chief said on Monday the alliance could only deploy to Iraq if Washington sought a new resolution at the United Nations - something the Pentagon may resist.