President Bush welcomed seven former Communist countries into NATO yesterday, pressing the alliance's boundaries farther into what once was Warsaw Pact territory and emphasizing its post-Cold War rebirth as a partnership aimed increasingly at fighting terrorism in Europe and beyond.

The expansion -- the second time the alliance has added members since the Soviet Union fell -- comes as a changing NATO prepares to send more forces into Afghanistan, considers a future role in Iraq, and works with nations in North Africa and elsewhere to thwart terrorist organizations.

"Terrorists hate everything this alliance stands for," Bush said in a White House ceremony with representatives of the seven nations. "They despise our freedom. They fear our unity. They seek to divide us. They will fail. We will not be divided. We will never bow to the violence of a few."

The relatively young democracies that joined the North Atlantic Treaty Organization yesterday included three former Soviet republics -- the Baltic states of Estonia, Latvia and Lithuania -- and three members of the former Warsaw Pact: Bulgaria, Romania and Slovakia. The seventh, Slovenia, was part of the former Yugoslavia. The invitation to join the alliance was extended at the NATO summit in Prague in November 2002 and was approved unanimously by the U.S. Senate last May.

The expansion of NATO from 19 to 26 countries tips the balance of the Atlantic alliance further eastward -- and tends to make the group as a whole more sympathetic to U.S. foreign policy. The seven, for example, backed Bush's move toward war in Iraq early last year, even as original NATO members France and Germany opposed him.

Bush pointedly noted in his remarks that all seven nations are playing supporting roles for U.S.-led military operations in Iraq or Afghanistan. Bulgaria, he said, provided refueling facilities for aircraft during the 2001 Afghan campaign and also has sent more than 400 soldiers to Iraq. Military engineers from Estonia and Latvia are helping clear explosives in Iraq, and forces from Lithuania and Slovakia also have served there, he said. Romanian and Slovenian troops have deployed to Afghanistan, he added.

"They understand our cause in Afghanistan and in Iraq because tyranny for them is still a fresh memory," said Bush, whose statements included a dose of Reagan-era anti-Soviet rhetoric. "When NATO was founded, the people of these seven nations were captives to an empire."

The alliance's growing roster has been eyed warily by Russia, which also expressed alarm at NATO's first expansion in 1999, when the alliance welcomed the Czech Republic, Hungary and Poland.

NATO Secretary General Jaap de Hoop Scheffer told a group of reporters Monday that fighter planes would begin "air policing" over the Baltic states at the moment their NATO membership took effect -- and that he explained the policy to Russian Foreign Minister Sergei Lavrov two weeks ago.

"I think that the Russian Federation has very well understood that NATO has, of course, no ulterior motives by air-policing its airspace," de Hoop Scheffer said, according to the Associated Press, adding that NATO has a solid relationship with Russia. "I think that NATO and Russia will further build on this partnership."

Russian officials have said they will respond if NATO's eastward expansion begins to look threatening. "If NATO believes that there is any need for such protection in the Baltic region, Russia reserves the right to draw its own conclusions from it and, if necessary, to act accordingly," Russian Foreign Ministry spokesman Alexander Yakovenko was quoted as saying by Itar-Tass, the AP said.

James Steinberg, a former deputy national security adviser, called yesterday's milestone "really quite a remarkable moment, because it is the completion of a project that dates back to the 1940s, when the Iron Curtain came down," cutting off Eastern Europe from the West.

Former Pentagon policy official James Bodner said the day was especially significant because it was part of a U.S. effort to provide incentives to eastern and southeastern European states to act democratically. Having the states adjoining the Balkans become peaceable and stable was by no means guaranteed when NATO began to contemplate expanding, he said. "A decade ago in Southeast Europe we almost had things unravel," he said.

Bush hinted at a third round of expansion of the 55-year-old security organization, noting that the prime ministers of three NATO aspirants -- Albania, Croatia and Macedonia -- also were in attendance yesterday. All three "are also contributing in Afghanistan or Iraq, proving their mettle as they aspire to NATO membership," Bush said. "The door to NATO will remain open until the whole of Europe is united in freedom and in peace," he added.

NATO might play a greater role in Iraq if authorized to do so by the United Nations Security Council, Scheffer told reporters yesterday before the ceremony. Currently the alliance provides some logistical and communications support to the Polish-led multinational division in southern Iraq, but otherwise it has steered clear of involvement.

"I think that the NATO allies would enter that discussion with a positive attitude, which could mean that NATO, as far as command is concerned, could participate or could take over a certain part of the stabilization force."