At the signing ceremony of the North Atlantic Treaty on April 4, 1949, then-United States President Harry Truman remarked that if this “simple document” had existed in 1914 and 1939, “it would have prevented the acts of aggression which led to two world wars.” The newly established alliance, he suggested, would ensure peace by deterring potential aggressors.

The success of this collective security strategy is reflected in the number of countries that have taken advantage of NATO’s open-door policy. Since 1949, the alliance has expanded from 12 to 32 members, with Finland and Sweden being the most recent additions. Now, member countries must commit to admitting Ukraine.

Bringing Kyiv into NATO is important for several reasons. For starters, any doubt about the alliance’s commitment to defend current or prospective members invites aggression. Russia is a case in point. The decision to shelve Ukraine and Georgia’s membership aspirations during the 2008 Bucharest summit led to Russia’s invasion of Georgia later that year. Similarly, turning a blind eye to Moscow’s 2014 annexation of Crimea resulted in the full-scale invasion of Ukraine in 2022.