Finland and Sweden formally applied for NATO membership on Wednesday, a move that, if approved, would fundamentally transform Northern Europe’s security landscape and provide the alliance with a valuable edge against Russia in the aftermath of Moscow’s invasion of Ukraine.

The two Nordic countries’ bid to join the North Atlantic Treaty Organization deviates from a decades-long defense strategy in which they balance political and security partnerships with other Western nations while remaining outside of formal military alliances.

According to live footage broadcast on Finnish television, Finnish Ambassador to NATO Klaus Korhonen and his Swedish counterpart Axel Wernhoff personally handed the applications to NATO Secretary-General Jens Stoltenberg in Brussels on Wednesday morning.

Finland and Sweden are already NATO allies, working together on missions and exercises. However, their formal accession would fill the largest remaining gap in NATO’s map of Europe, and it would do so in a volatile part of the continent. With Arctic ice melting and shipping increasing near the North Pole, the region’s air and naval activity has increased in recent years. The Baltic Sea, which is Russia’s shortest route to the Atlantic Ocean, would be dominated by NATO allies.

The announcement was expected after the Finnish and Swedish ruling parties supported NATO applications over the weekend, capping weeks of rapid political decision-making and a significant shift in public opinion among Finns and Swedes who now overwhelmingly support joining the alliance, hoping that such a move will deter Russia from any aggression on their soil.

Even for countries as well-suited as Finland and Sweden, gaining NATO membership is a multi-step process. Following the declaration of intent to join, an applicant country will meet with teams of NATO experts in Brussels to ensure that the prerequisites for membership have been met.

Then it is up to all current NATO members to ratify the accession protocols that allow the invited countries to become parties to the so-called Washington Treaty, which serves as NATO’s legal foundation. Most countries require parliamentary approval to ratify the protocols, and while many nations have stated their willingness to accept Sweden and Finland as members as soon as possible, political circumstances in individual countries may slow the process.

Recep Tayyip Erdogan, Turkey’s president, has stated that he opposes Sweden and Finland joining NATO due to the presence of alleged Kurdish militants in both countries. Turkish officials have recently demanded that Nordic countries crack down on networks linked to the Kurdistan Workers’ Party (PKK), a group that has waged a long-running insurgency against the Turkish state and is designated as a terrorist organization by Turkey, the European Union, and the United States.

On Wednesday, Mr. Erdogan told members of his party that he was concerned about Sweden’s alleged refusal to hand over alleged terrorists.

In recent months, the Turkish government has also sought to use its role in militarily supporting Ukraine while facilitating Russian-Ukrainian peace talks to put pressure on Western countries to make concessions. Turkish officials are hoping that President Biden will persuade Congress to approve the sale of a new fleet of F-16 fighter planes to Ankara.

Despite being a close NATO partner, Finland has insisted on remaining nonaligned, largely to appease Russia, with which it shares an 830-mile border and fought a bloody war eight decades ago that the country still remembers bitterly.

Sweden’s opposition to NATO membership has been more ideological, stemming from a desire to be a global peacemaker and a vocal advocate for nuclear disarmament. Sweden has not fought a domestic war in two centuries, since Russia invaded the country in 1808, launching a war that resulted in the annexation of what is now Finland. Still, both countries are well-prepared for NATO membership. Since 1995, their forces have participated in NATO operations and ensured their military equipment can function with the alliance’s gear.