In contrast to what happened later in Africa and Asia, the Americas were not just occupied, but largely repeopled, by Europe. With the exceptions of Canada and a few Caribbean colonies, the European settler-states in the Americas broke free from Britain, Spain, and Portugal during the late eighteenth and early nineteenth centuries. As happened later in Africa and Asia, this first wave of decolonisation was achieved by taking on the form of the European state and, as this was done, the conditions for standard RSCs came into being.
Despite recurrent hemispheric projects, the Americas are not one RSC. North and South America have different security dynamics and connections are highly asymmetrical. US engagement in Latin America is a classical case of a complex containing a great power impinging on a neighbouring one without great powers. South America has only infrequently been the primary security concern of the United States, and in South America the driving security dynamics are mostly regional, not US-orientated. So even recurring US involvement does not justify seeing the Americas as one RSC. The USA is an (important) external actor in South America, and South America has some spillover security effects in North America, but most issues that upset Canada will be of minor relevance to Brazil and vice versa.
It is clear that North America is one RSC, and equally clear how to delineate it towards north, east, and west, but it is less clear where it ends to the south. Traditionally the concept of North America did not include Mexico, which was grouped on cultural and linguistic grounds with Latin and/or Central America. Today it is much more common to see Mexico counted as North America. This is not only because of NAFTA–rather NAFTA reflects a change in larger patterns.