From Vietnam to Afghanistan, U.S. leaders have had great difficulty disentangling the United States from faraway military interventions. William McKinley's 1898 decision to annex the Philippines reveals why, through a phenomenon called the “meddler's trap.” The meddler's trap denotes a situation of self-entanglement, whereby a leader inadvertently creates a problem through military intervention, feels they can solve it, and values solving the new problem more because of the initial intervention. The inflated valuation is driven by a cognitive bias called the endowment effect, according to which individuals tend to overvalue goods they feel they own. A military intervention causes a feeling of ownership of the foreign territory, triggering the endowment effect. Following the U.S. victory in Manila during the War of 1898, McKinley doubted Filipino civilizational capacity to self-govern, believed that a U.S. departure from the Philippines would cause chaos and great power war, and believed that U.S. governance could forestall that outcome. Because he had already deployed troops to the Philippines, McKinley also felt ownership over them, and this endowment effect inflated his valuation of the archipelago. Together, these mutually reinforcing beliefs produced the meddler's trap and the United States’ largest annexation outside its hemisphere.