Europe's experience should give ambitious economic unions elsewhere in the world pause. As we reflected on the European case study, we traced the evolution of six other regional blocs — Mercosur, the Gulf Cooperation Council (GCC), the Association of Southeast Asian Nations (ASEAN), the East African Community, the Eurasian Economic Union and the North American Free Trade Agreement (NAFTA) — assessing the geopolitical forces that first drew them together and the underlying constraints that will ultimately keep their integrationist appetites in check. More often than not, the spectrum of integration among states begins with free trade agreements, which can then be upgraded to customs unions with common external tariffs and common markets that enable the free flow of labor, goods, services and capital. From there, blocs can evolve into more extreme forms of integration, such as monetary unions or political federations.