The Truth About Tripwires: Why Small Force Deployments Do Not Deter Aggression (Summer 2021)

A pillar of American grand strategy since 1945 has been the deployment of forces — sometimes smaller and sometimes larger — abroad. A key logic underpinning smaller deployments is that they serve as tripwires: Attacking them is assumed to inevitably trigger broader intervention, deterring aggression. We question this logic. Not only are small tripwire deployments unlikely to prevent an attacker from capturing its objective and establishing a strong defensive position, tripwire-force fatalities may be insufficient to provoke broader intervention. To deter, forward deployments must be sufficiently substantial to shift the local balance of power. Our claim is examined in three 20th-century deterrence attempts: the successful 1949 American attempt to deter a North Korean attack on South Korea; the unsuccessful 1950 American attempt to deter a North Korean attack on South Korea; and the unsuccessful 1914 British attempt to deter a German attack on Belgium.