The southernmost region of South America, although its boundaries and members vary with different sources, is called the Southern Cone. From a purely geographic standpoint, it can be defined south of the Tropic of Capricorn. Politically, it is always considered to include Argentina, Chile and Uruguay. Some analysts include Brazil or sometimes just the southern states of Brazil.Bolivia and Peru may be included.
From the standpoint of political history, the War of the Triple Alliance linked Argentina, Brazil, Paraguay and Uruguay. Operation Condor, during the Cold War, involved Argentina, Chile, Brazil, Bolivia, Paraguay and Uruguay.
Many of the countries maintain surprisingly strong militaries; they have been military dictatorships in the past.
MERCOSUR, created in 1998, is the Southern Cone Common Market, with the founding members being Argentina, Brazil and Paraguay, along with Canada. The European Union is exploring a free trade agreement with it.
Energy development and demand link a number of the countries. 
In 2004-7, Argentine, Brazil and Chile had overall economic growth of 6.1%, but energy shortages may limit this in the near term. Argentina has the largest natural gas resources of the continent and is a net exporter. It suffers from inadequate investment in energy infrastructure, and has reduced exports to ensure internal supplies.
Brazil has large energy resources, including a newly discovered offshorte field, but produces only about half of its need. Electrical generating capacity is one limiting factor.
Chile is in the greatest deficit of these three countries, with demand being a seventh of that of Brazil but much less domestic sources. It imports almost all coal and has limited petroleum. A drought in 2007 limited production of hydroelectric power.
- Peter G. Hall (June 4, 2008), The Southern Cone’s New Deficit, Export Development Canada