Triple Frontier

The Triple Frontier (, ) is a tri-border area along the junction of Paraguay, Argentina and Brazil, where the Iguazú and Paraná rivers converge. Near the confluence are the cities of Ciudad del Este (Paraguay); Puerto Iguazú (Argentina) and Foz do Iguaçu (Brazil). This area is near Iguazú Falls and the Itaipú hydroelectric plant.
Population.
The population in the Triple Frontier is concentrated in three border cities and in the Paraguayan metropolitan region. Of these, the largest is Ciudad del Este in Paraguay, which in 2018 had a population of 299,255, while the smallest with a population of 82,000 is Puerto Iguazú, Argentina. The tourist-centric Brazilian city Foz do Iguaçu has a population of 300,000. In the metropolitan region of the Paraguayan side, Porto Franco has a population in 2018 of 98,805; Hernandarias, 79,036; and Minga Guazú, 86,755. Altogether, the population of the Triple Frontier adds up to about 950,000 people. The Arab and other Asian immigrant communities, which make up an important part of the urban population in the Tri-Border Area, are estimated to number approximately 100,000.
Tourism.
The Triple Frontier is an important tourist area, within the touristic subregion of the Región de las Aguas Grandes. Visitors can see the Tancredo Neves bridge, which connects the Argentine city of Puerto Iguazú and its Brazilian neighbor, Foz do Iguaçu. At the convergence of the borders, each of the three bordering countries has erected an obelisk, painted in the national colors of the country in which it is located. All three countries can be seen from each of the obelisks.
The Guarani Aquifer is arguably the biggest reservoir of fresh, potable water in the world—right under Triple Border soil (Brazil, Argentina and Paraguay). The majority (71%) of its 1.2 million square kilometers lies in Brazil. 
US Security concerns.
The George W. Bush administration cited "clear examples" of Islamic groups in the tri-border region in 2002 that "finance terrorist activities." Hezbollah, and Hamas were believed at the time to operate in the Triple Frontier.
The particular geography of the border region, rampant political corruption and weak judicial system make it very difficult to monitor organized crime and the illicit activities connected with it. To U.S. officials and law enforcement familiar with the region, "Iranian-backed Hezbollah militia have been fostering a well-financed force of Islamist radicals in the region".
A counter-terrorism expert with the Pentagon's National Security Study Group described the Tri-border in 2007 as "the most important base for Hezbollah outside Lebanon itself, home to a community of dangerous fanatics that send their money to financially support Hezbollah." Of the 25,000 Lebanese Arabs who live in the region, not all support terrorism, but many openly acknowledge they send money to Hezbollah and that Shiite mosques have "an obligation to finance it".
The Paraguayan authorities say they have evidence that money is being sent to organizations with terrorist connections because of the amount of money leaving Paraguay for the Middle East, said Carlos Altemburger, Chief of the Department for the Prevention and Investigation of Terrorism in Paraguay. In response to the situation, Paraguay approved the entry of 400 US soldiers "for joint military exercises, such as programs on fighting urban terrorists, public security and humanitarian assistance", according to "The Washington Post". However, in October 2006 Paraguay decided not to renew a defense-cooperation agreement.
Foz do Iguaçú tourist groups dispute the reports of terrorist activity in the region, as has the U.S. State Department.
Since 1996, the Tripartite Command of the Triple Frontier, which coordinates monitoring tasks among the three bordering countries, has been operating from Foz do Iguaçu. The 3+1 Group was created between Argentina, Brazil, Paraguay, and the United States (the "1"), in 2002 to further strengthen security in the region. In 2005, the governments of the three nations stated they would set up a joint intelligence center in Foz do Iguaçú specifically to monitor the situation.