The North Atlantic Treaty Organization (NATO) is a **transatlantic military alliance of 32 member countries from Europe and North America**, established in 1949 to provide collective defense against Soviet aggression during the Cold War[1][2]. Its founding treaty commits members to democracy, individual liberty, the rule of law, and peaceful dispute resolution, with a crucial principle of collective defense enshrined in Article 5: an attack against one member is considered an attack against all[2][3]. Since the Cold War's end, NATO has broadened its mission beyond territorial defense, engaging in operations worldwide, including Afghanistan and Libya, to promote global security[1]. The alliance has steadily expanded, adding 16 members since 1999, with Finland and Sweden joining most recently in 2023 and 2024, respectively[2][3]. NATO remains pivotal in **deterring Russian aggression**, especially after Russia's 2022 invasion of Ukraine, which has intensified security concerns in Europe and led to increased military and economic support for Ukraine[1]. This conflict has driven NATO members to reconsider their defense commitments and spending, with the 2025 summit focusing on raising defense spending targets and strengthening collective deterrence[1][4][5]. The alliance maintains an "open door" policy allowing European countries to join if they meet certain criteria, such as spending at least 2% of GDP on defense, though discussions at the 2025 summit may raise this target to around 3.5% or higher[4][5]. Notably, NATO includes three nuclear-armed states (the U.S., the U.K., and France), enhancing its strategic deterrence capabilities[2]. The alliance’s ability to adapt to new security challenges—from hybrid warfare to cyber threats—keeps it central to global security architecture, especially amid rising geopolitical tensions and risks of armed conflict identified for 2025[4]. In summary, NATO is a **cornerstone of transatlantic security