BUCHAREST, Romania — President Bush set the stage yesterday for a contentious three-day North Atlantic Treaty Organization summit, where disputes over missile defense, the war in Afghanistan, and expansion into former Soviet territory likely will test the unity of the 26-member alliance.

As leaders converged on this Balkan capital, two decades after the ouster and execution of its communist dictator opened doors to the West, Mr. Bush called on the Atlantic alliance to do more to transform itself for a new era of more disparate threats. But his calls for additional NATO troops for Afghanistan and membership roadmaps for the former Soviet republics of Ukraine and Georgia provoked deep skepticism and face an uncertain outcome.

“NATO is no longer a static alliance focused on defending Europe from a Soviet tank invasion,” the president said yesterday morning at a parallel conference sponsored by the German Marshall Fund of the United States, Chatham House, and the Romanian Foreign Ministry. “It is now an expeditionary alliance that is sending its forces across the world to help secure a future of freedom and peace for millions.”

Mr. Bush then headed off to the Black Sea resort of Neptun for meetings with President Basescu of Romania before the NATO summit formally opened yesterday evening with an official dinner. During a news conference Mr. Bush pressed his case for NATO allies to send more forces to Afghanistan. “We expect our NATO allies to shoulder the burden necessary to succeed,” he said.

The alliance is poised to invite new members at its session today, extending further into the once-troubled Balkans by admitting Croatia, Macedonia, and Albania, three nations once caught up in the swirl of conflicts that played out through the 1990s following the breakup of Yugoslavia. The alliance may also agree to open preliminary discussions about future membership with Bosnia and Montenegro, two other former Yugoslav republics.

NATO members are deeply divided, however, over bids by Ukraine and Georgia for what are known as membership action plans, a years-long process of civilian and military reforms intended to lead to eventual admission into the alliance. Russia, which is not part of the alliance, strongly opposes efforts by Ukraine and Georgia to join NATO. Germany and France have declared they will block the Ukrainian and Georgian applications, while America, Canada, and nine Eastern European members are pushing strongly for them.

“These two nations inspired the world with their Rose and Orange revolutions,” Mr. Bush said, referring to democratic uprisings in Georgia in 2003 and Ukraine in 2004. “And now they’re working to consolidate their democratic gains and cement their independence. Welcoming them … into the membership action plan would send a signal to their citizens that if they continue on the path to democracy and reform they will be welcomed into the institutions of Europe.”

Mr. Bush has signaled in the past two days that he is not looking for a face-saving compromise and would rather press the confrontation even if it ruptures the summit. Before arriving here, he stopped in Ukraine for a high-profile display of support, declaring that “Russia will not have a veto over what happens in Bucharest” and dismissing any suggestions of a deal with Moscow. “There’s no trade-offs, period.”