Finland, with its 830-mile border with Russia, offers NATO a much stronger position to deter aggression, offering access to a strong military, as well as Finnish airspace, ports and sea lanes. NATO will also be better able to defend the Baltic nations and the Arctic, said Matti Pesu, a security expert with the Finnish Institute of International Affairs.

“Finland will contribute to allied deterrence and defense, and NATO’s area of operations will significantly grow,” Mr. Pesu said.

The addition of Finland will double NATO’s border with Russia, but Finnish experts note that Russia has had to move most of its military forces away from there to sustain its war in Ukraine.

And Finland is already within range of Russian forces and nuclear-armed missiles based in the Kola Peninsula and St. Petersburg, “so Finnish membership won’t be a game changer,” Mr. Pesu said.

Nevertheless, the Russian Embassy in Sweden on Wednesday threatened both Finland and Sweden with military retaliation if they join NATO. Sweden’s membership application is being blocked by Turkey and Hungary.

“If anyone still believes that this will somehow improve Europe’s security, you can be sure that the new members of the hostile bloc will become a legitimate target for Russia’s retaliatory measures, including military ones,” the embassy said in a posting on its Facebook page.

With Turkey’s ratification, Finland now faces only paperwork to become a NATO member. There will be an exchange of letters and the filing of Finland’s accession documents, already complete, with the State Department in Washington. The United States serves as the depository of NATO under the alliance’s founding treaty.

Finland and Sweden, its neighbor and close security partner to the west, had hoped to join NATO “hand in hand.” But Sweden’s membership application continues to be blocked by both Turkey and Hungary. Accepting a new member into the alliance requires unanimous agreement by all 30 member countries. Hungary approved Finland’s bid on Monday, leaving consent from Turkey  the last to be  needed.

While Finnish officials say they will continue to push for rapid accession for Sweden, the Swedes have said that they accept Finland’s decision to join alone.

Still, it will be some time before Finland and NATO fully integrate their defense plans. Finland must decide, for example, whether it needs or will accept foreign troops or even nuclear weapons on its territory, Mr. Pesu said.

National elections are taking place in Finland on Sunday, and final membership may help the effort of Prime Minister Sanna Marin and her Social Democrats to remain in power. The race is tight, with the three largest parties essentially tied in the latest polls.