BRUSSELS — Turkey blocked the start of Finland and Sweden’s accession talks to NATO on Wednesday shortly after the Nordic nations submitted their formal applications, a signal of what could be a bumpy process to expand the alliance and reshape Europe’s post-Cold War security architecture.
Turkey’s resistance deprived Secretary General Jens Stoltenberg of the consensus he needed to move forward with the membership process. It also put a damper on a historic moment for two countries that held fast to military nonalignment until Russia’s invasion of Ukraine upended their thinking about security.
At a meeting of NATO ambassadors, Turkey said that it still needed to work through some issues related to Finland and Sweden joining the alliance, according to two officials familiar with the discussion, who spoke on condition of anonymity to discuss the sensitive closed-door talks.
Turkish President Recep Tayyip Erdogan has objected to Sweden’s granting of asylum to members of Turkey’s Kurdish minority group, and he has indicated that he will seek other concessions if he is to allow the expansion to go forward.
NATO diplomats still widely believe Turkey will eventually waive its objections and allow the expansion, which would double the alliance’s land border with Russia. But a process that was already expected to take months could be slower and more complicated than other alliance members had hoped.
A second source of uncertainty is whether Russian President Vladimir Putin will retaliate. Finland and Sweden are in some ways most vulnerable to Russian attack during the period before they actually join, since they are still not covered by NATO’s mutual defense guarantees.
European officials and diplomats said the two countries are prepared for hybrid or clandestine attacks. Several allies have also offered assurances that Finland and Sweden could expect protection in the interim period.
Stoltenberg called the applications submitted Wednesday a “historic step.”
“I warmly welcome the requests by Finland and Sweden to join NATO,” he said at a news conference in Brussels with ambassadors from both countries. “You are our closest partners, and your membership in NATO would increase our shared security.”
Swedish Prime Minister Magdalena Andersson and Finnish President Sauli Niinisto are scheduled to appear Thursday at the White House, where President Biden is expected to show his support.
The two new members would bring NATO’s full force to the far north and bolster its presence in the Baltic Sea region. The alliance would gain two sophisticated militaries with deep experience operating near Russia’s frontier. Sweden also holds the strategically important island of Gotland, just 200 miles from the Russian military in Kaliningrad.
Finland and Sweden didn’t consider themselves neutral before now. Militarily, they have been close NATO partners. Politically, they are members of the European Union.
But thinking of themselves as nonaligned militarily has been an important part of their self-conception. Before Russia’s invasion of Ukraine, a majority of people in both countries said it was safer to be outside NATO. But the past months have seen a dramatic swing in public opinion.
“This is an extraordinary development given where we were in February,” said Anna Wieslander, director for Northern Europe at the Atlantic Council think tank.
“Russia wanted to turn back time, to go back to the Cold War, to fragment and weaken the West,” she continued. “Now, in May, we are here.”
Erdogan left the door open on Wednesday to approving the expansion, but he made clear that he wanted his concerns to be addressed by NATO and by Sweden and Finland.
“We are one of the countries that give the most support to the activities of the alliance, but this does not mean that we will unquestioningly say ‘yes’ to every proposal brought before us,” he told members of his political party in Ankara. “The expansion of NATO is meaningful for us, in proportion to the respect that is shown to our sensitivities.”
He said Turkey had asked Sweden to extradite “30 terrorists” — a reference to members of the Kurdistan Workers’ Party, or PKK, which the United States and Turkey have designated a terrorist organization.
“They said, ‘We won’t give them.’ You will not hand over terrorists to us, but you will ask us to get up and join NATO,” he said. “NATO is a security formation, a security organization, so we cannot say ‘yes’ to depriving this security organization of security.”
Other members of the alliance expressed support for the bid on Wednesday. British Prime Minister Boris Johnson called it a “historic day for our alliance and the world.”
“Not long ago nobody would have predicted this step, but Putin’s appalling ambitions have transformed the geopolitical contours of our continent,” he wrote on Twitter.
In a statement, Canadian Prime Minister Justin Trudeau said Canada will “wholeheartedly endorse their application without reservation.”
“Their militaries are highly qualified and capable, and we have full confidence in the ability of both countries to integrate rapidly into NATO,” the statement said.
Putin cited the threat of NATO expansion among the rationales for his unprovoked invasion of Ukraine. And Russian officials have warned of “consequences” for Finland and Sweden joining. But they have offered more muted rhetoric in recent days.
Putin said Monday that Finland and Sweden’s entry into NATO did not represent an imminent danger to Russia, but he warned that a military buildup in either country could change that assessment.