The aim of this thesis is to examine how a membership in a defense alliance affect the Naval forces. Three naval theories are used to examine organization, reach and adaptation to needs. A case study with Poland and Spain has been conducted. Both countries Naval forces have been analyzed before and after membership in NATO 1982 respectively 1999. The result shows a significant difference when it comes to reach. Reach is used to explain the ability to operate in different types of geographical areas. Both countries have become more of Blue-water Navies after membership in NATO. It is also a tendency when it comes to adaption to needs, both Navies have moved towards a risk fleet after becoming a member in NATO. The conclusion is that a membership in a defense alliance affect the Naval Forces when it comes to reach and adaption to needs.