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Alliances and ballistic-missile defense (BMD) are both significant elements of U.S. security policy, but the emphasis on each may be changing. Since the end of World War II, the global network of allies of the United States has been a strategic cornerstone.1 The Trump administration's National Security Strategy reaffirms the vital role of alliances in U.S. security, but President Trump's interactions with traditional U.S. partners suggest a new degree of fluidity in these longstanding relationships.2 BMD's importance surged in the first half of the Trump administration, particularly after North Korea tested ballistic missiles that could threaten the U.S. homeland. In response, the U.S. Congress increased the Missile Defense Agency's funding by over one-third, from $8.2 billion in 2017 to $11.5 billion in 2018.3 How might an increased emphasis on BMD affect U.S. alliance relationships?
U.S. defense policy contends that BMD strengthens alliances. The historical record, however, is mixed. While BMD has bolstered alliances at times, at other times it has exacerbated allies' doubts about U.S. commitment. Why have allies responded so differently to BMD and what are the implications for contemporary U.S. policy? Variations in threat perceptions, relative dependence and vulnerability, and expectations of U.S. commitment could cause an ally to perceive that U.S. BMD increases the risk of abandonment or entrapment-meaning that the United States might either shirk an alliance obligation or drag an ally into a war, respectively.4 When determining BMD policy and investments, U.S. leaders should consider not only expected defensive benefits but also how their decisions might affect allies' perceptions of alliance cohesion and credibility.
After providing theoretical background on alliances and the concepts of abandonment and entrapment, this article will review how U.S. policy expects BMD to benefit alliances. It then tests this policy framework against two sets of case studies. The first set examines the earliest U.S. BMD system-Sentinel-in the late 1960s and its impact on alliance relationships with Japan and European NATO allies. The second set considers BMD since the end of the Cold War and again evaluates its effect on the Japanese and NATO alliances, and adds South Korea. The article lastly recommends how these lessons from the past and the present can better inform contemporary U.S. BMD policy.
ALLIANCE THEORY: ABANDONMENT AND ENTRAPMENT
States form alliances...