The latest U.S. National Security Strategy suggests that a key American objective in Europe should be to dispel the belief—and prevent the outcome—of NATO functioning as an alliance that expands indefinitely. The Trump administration signaled skepticism toward continued enlargement by opposing Ukraine’s bid for membership. In contrast, many European allies still uphold NATO’s formal position that accession remains open and that Ukraine is on an “irreversible” trajectory toward joining the alliance.
This divergence risks creating serious friction across the Atlantic, though such an outcome is not inevitable. European partners should acknowledge that NATO’s enlargement has increasingly produced unintended consequences. The alliance’s growth from 16 to 32 members since the Cold War underscores its enduring appeal as a collective security structure. Yet allowing expansion to continue without strategic reassessment threatens to undermine earlier successes.
NATO’s post–Cold War expansion was initially grounded in the principle that European nations should be free to choose their security arrangements. This approach worked while the interests of major stakeholders—including Russia—could be managed. In 1997, NATO and Russia agreed on foundational guidelines for cooperation before Poland, Hungary, and the Czech Republic were formally invited to join. Their admission in 1999, followed by seven additional Central and Eastern European countries in 2004, including the Baltic states, occurred without strong Russian retaliation. Many interpreted Moscow’s muted response as a sign of growing acceptance.
In reality, Russia never fully reconciled itself to NATO’s eastward advance. Even though the military footprint in new member states remained limited—with no permanent deployment of large combat forces or nuclear weapons—the incorporation of former Warsaw Pact members and former Soviet republics represented a major geopolitical rebalancing. Compounding Russian concerns was NATO’s framing of enlargement as an open-ended process, leaving Moscow to fear the complete erosion of its regional influence.
The critical turning point came in 2008, when NATO announced that Ukraine and Georgia would eventually become members. Although no timetable was established, this declaration crossed what many in Moscow viewed as an absolute red line—a concern previously highlighted by then–U.S. ambassador William Burns. Coming shortly after President Vladimir Putin publicly criticized Western disregard for Russian security interests, the move was seen as a direct challenge to Russia’s perceived sphere of influence.
As Russia became more authoritarian and relations with the West worsened, some analysts argued that NATO should limit membership to states it could credibly defend. Yet adopting this position would have stranded aspiring countries in a strategic vacuum between NATO and Russia, while also granting Moscow indirect influence over alliance decisions. As a result, the West continued to portray enlargement as a moral obligation—a corrective to Cold War divisions that should not be questioned.
Persisting with an unconditional open-door policy now places NATO in a strategic bind. Any negotiated resolution to Russia’s war against Ukraine would be far harder to achieve. Moreover, extending security commitments to countries that NATO may be unwilling or unable to defend carries growing risks. While refusing membership to Ukraine and other candidates may appear to violate their sovereign right to choose alliances, the West should not feel compelled to maintain a policy that increasingly generates instability. What began as a principled strategy now risks producing ethically troubling outcomes in a changed geopolitical environment.
The real failure of NATO enlargement is not that it began, but that the alliance never seriously debated when—or where—it should stop.
