Founded in 1949, NATO is a 32-member mutual-defense alliance. Article 5 — the treaty's collective-defense clause — has been invoked only once: by the U.S. after the September 11 attacks.
Members pledged in 2014 to spend at least 2% of GDP on defense. As of 2024, NATO reported 23 of 32 members meeting the target, up from 6 in 2021.
Sweden and Finland joined NATO in 2023 and 2024 respectively, following Russia's 2022 invasion of Ukraine — the alliance's first new members in over a decade.
Supporters argue NATO has prevented major-power war in Europe for 75 years and that the U.S. benefits from collective deterrence at relatively modest cost. Critics argue Europe free-rides on American defense spending and that NATO expansion has provoked rather than deterred Russia.
Estimates of the U.S. share of NATO's combined defense spending range from about 67% to 71%, depending on how indirect costs are counted.