A politico-military alliance of the countries from both sides of the Atlantic Ocean founded in 1949 and originally serving the purpose of defending its member states against a possible Soviet attack. The United States is a natural leader of the organisation, the core principle of which is stated in the Article 5 of the Washington Treaty, which declares that an armed attack against any member state will be considered as an attack against them all. NATO is one of the most successful defensive organisations in modern history thanks to its 70-year-long existence and gradual enlargement to post-communist countries after the end of the Cold War. During its lifetime, the Alliance had to frequently adapt to a changing security environment and new challenges, including the USSR, the instability in the Balkans, terrorism and the rise of China.