Who—and what—moves from one country to another has real implications for security studies, international relations, and the ideal of democracy.

In summer 2014, US agencies responsible for the border with Mexico were overwhelmed by tens of thousands of unaccompanied children arriving from Central America. Unprepared to address this unexpected kind of migrant, the US government deployed troops to carry out a new border mission: the feeding, care, and housing of this wave of children.

This event highlights the complex social, economic, and political issues that arise along borders. In American Crossings, nine scholars consider the complicated modern history of borders in the Western Hemisphere, examining borders as geopolitical boundaries, key locations for internal security, spaces for international trade, and areas where national and community identities are defined.

Among the provocative questions raised are: Why are Peru and Chile inclined to legalize territory disputes through the International Court of Justice, undermining their militaries? Why has economic integration in the "Tri-Border Area" of Argentina, Brazil, and Paraguay increased illicit trade supporting transnational terrorist groups? And how has a weak Ecuadorian presence at the EcuadorColombia border encouraged Colombian guerrillas to enforce the international borderline?

Maiah Jaskoski is assistant professor in the Department of Politics and International Affairs at Northern Arizona University and author of Military Politics and Democracy in the Andes. Arturo C. Sotomayor is associate professor in the Department of Political Science and Geography at the University of Texas at San Antonio and author of The Myth of the Democratic Peacekeeper: Civil-Military Relations and the United Nations. Harold A. Trinkunas is director of the Latin America Initiative at The Brookings Institution and author of Crafting Civilian Control of the Military in Venezuela: A Comparative Perspective.