British Prime Minister Boris Johnson, here seen addressing a press conference with Finnish President Sauli Niinisto in Helsinki, Finland, on Wednesday, signed a security pact with Finland just ahead of the European nation’s decision on whether to apply for NATO membership.Frank Augstein | Afp | Getty ImagesA decision by Finland and Sweden to join NATO will improve deterrence against Russia in northern Europe and add to the U.S.-led military alliance’s security, one analyst from Atlantic Council told CNBC on Thursday.Her comments came right before Finland’s President Sauli Niinisto and Prime Minister Sanna Marin announced their country should apply to join the North Atlantic Treaty Organization “without delay.” “NATO membership would strengthen Finland’s security. As a member of NATO, Finland would strengthen the entire defense alliance,” they said in a joint statement.A similar announcement from Sweden is expected later this week.”Sweden and Finland joining NATO will increase deterrence in the region because of the assets they will provide the alliance. And [because of] the ability to plan for Northern Europe’s security in a comprehensive manner,” said the Atlantic Council’s Northern Europe director Anna Weislander. Membership of the political and military alliance will be a historic decision for Finland, which shares a 1,300-kilometer border with Russia. The Nordic nation adopted neutrality after its defeat by the Soviet Union in World War Two. Sweden, too, has not been part of any military alliances for more than 200 years.Weislander said both the countries were well prepared to meet the oft-repeated political and military threats by Russian President Vladimir Putin who opposes their membership.”President Putin and Russia have several times already declared that there will be military and political consequences,” she said on “Capital Connection,” adding she expected more of such warnings in the coming days.”We have prepared. We have moved military installations… and expect [Russian] cyber attacks, electronic jamming or more airspace intrusions,” she said, noting that both Sweden and Finland were members of the European Union.”Sweden and Finland are solid democracies with sophisticated economies and will, therefore, also contribute to the underlying values of the alliance,” Weislander said.The armed forces of both countries enjoy high compatibility with NATO members states too, she added.’Interoperability’ with NATO members”Finland is already a security provider. It has a strong military, it is small but technically sophisticated. It can not only defend itself despite a long border with Russia but it has also, alongside Sweden, worked with NATO on international missions from the Balkans to Afghanistan. And they have practiced extensively,” Weislander said.Both countries are “operable” as NATO members, she said.”[They] have worked with NATO since the mid-1990s in international missions,” the analyst said, citing those in the Balkans, Afghanistan and Libya. “They have also participated in advanced exercises with NATO, such as Trident Juncture. And there were hundreds of other exercises.””So their interoperability is not a problem,” Weislander said.Sweden and Finland also cooperate extensively on defense with each other based on NATO standards, she added.If both countries join NATO, this would signal an “open door” NATO policy, Weislander said.”When countries are ready to join and can contribute to the security of the whole of the transatlantic area, then [NATO] will be ready to take on new members as well. There will be no closed door for NATO,” she said.