Although Denmark, Finland, Norway, and Sweden are ethnically and culturally similar, and all relied on neutrality to protect their sovereignty prior to the Second World War, Denmark and Norway sought protection from the North Atlantic Treaty Organization NATO after the war while Finland and Sweden still remain outside the Alliance. This thesis explores the reasons for these diverging paths before discussing what might cause Finland and Sweden to join NATO in the future. The thesis finds that recent Russian aggression has pushed Finland and Sweden closer than ever to seeking membership, and concludes that they will probably join the Alliance eventually, but only after their general elections in 2019 and 2018, respectively, at the earliest. Additionally, the thesis notes that Finland is a stronger proponent for non-alignment, while Sweden is increasingly pushing the duo toward NATO membership. Furthermore, the research indicates that the NATO Allies would welcome these two countries into the Alliance if they sought membership. U.S. foreign policy might most effectively encourage Finnish and Swedish membership by avoiding bilateral security guarantees with them, supporting fact-based public debates in the countries, and engaging with their governments.