Hannah Jakob Barrett

Alliances like NATO rest on an expectation of reciprocity; each member's obligation to defend its allies is conditional on those allies upholding their own commitments. This expectation extends beyond formal treaty obligations to include member states’ adherence to shared norms and commitment to equitable burden-sharing. The US is the primary architect and guarantor of transatlantic security, and European allies must trust it will use its military might to defend the continent. In the current era of transatlantic relations, President Donald Trump has not only expressed a desire to abandon NATO allies to Russian aggression, but has threatened to invade Greenland, a Danish territory that the US is formally bound to defend. Within states that rely on American security guarantees, the preferences of the mass public will play an essential role in defining which visions of security are deemed viable in the wake of US defection. This dissertation provides consistent evidence that US defection pressures allied publics to narrow their security interests, strengthen national defense, and limit mutual defense to a more exclusive in-group of states. US defection also pressures the allied public to abandon commitments to states outside the formal alliance, despite the strategic incentive to deter Russian aggression. Even in high-stakes deterrence scenarios, US defection suppresses the allied public's defiant instincts and makes them less willing to stand firm against powerful adversaries. The corrosive effect of US defection indicates that the micro-foundations of alliance cohesion are only durable so long as the most powerful state's commitments remain credible.  

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