The **North Atlantic Treaty Organization (NATO)** is a transatlantic military alliance founded in 1949 to secure peace in Europe and protect democratic nations from Soviet aggression. It began with 12 founding members and has expanded to include 32 countries across North America and Europe as of 2025, including recent additions Finland (2023) and Sweden (2024)[1][3]. NATO's core principle is collective defense, meaning an attack against one ally is considered an attack against all, enshrined in Article 5 of its founding treaty[3]. Originally established as a Cold War bulwark, NATO has evolved beyond its initial Euro-Atlantic focus, engaging in missions worldwide, such as in Afghanistan and Libya[1]. The alliance remains central to U.S.-Europe military cooperation and has adapted to new security challenges, particularly Russia’s 2022 invasion of Ukraine, which has significantly reshaped NATO’s priorities. This aggression has spurred increased defense spending, stronger collective deterrence efforts, and accelerated Ukraine’s path toward potential membership[1][4]. In June 2025, NATO held its first summit in the Netherlands, where leaders agreed on raising defense spending targets to 5% of GDP—split between 3.5% for core defense and 1.5% for related security costs, including support for Ukraine. This marked a historic commitment to bolster the alliance’s military capabilities amid evolving global threats. The summit also emphasized eliminating defense trade barriers and reaffirmed the ironclad commitment to collective defense[2][5]. NATO’s ongoing significance lies in its role as a strategic security platform fostering cooperation among member states, responding to geopolitical instability, and adapting to emerging threats like terrorism and cyber warfare[4]. Its "open door" policy continues to invite eligible European democracies to join, reinforcing its mission to safeguard peace, democracy, and shared technological and defense innovations across allied nations[1][4].