Isla Calero (English: Calero Island) is a Costa Rican island, the biggest island in Costa Rica, and the biggest along the San Juan River, which marks the border between Nicaragua and Costa Rica. The island lies between the San Juan (to the north and west), the Río Colorado of Costa Rica (to the south and southeast), and the Caribbean Sea (to the east and northeast). The entire island has an area of 151.6 km² (58.5 mi²).
In November 2010, the northern tip of Isla Calero, an ecological preserve, became the focus of a dispute and extensive media coverage when Nicaraguan troops occupied it. Both nations base their claims on their interpretations of three historic documents, the Cañas–Jerez Treaty, the Cleveland Award, and the Alexander Award issued by E.P. Alexander who acted as an arbitrator between the two countries in 1897. In December 2010, Nicaragua defended their claim to the OAS by indicating that Google Maps was the main source of their decision. Google maps quickly responded by correcting the error on the maps and saying "Google maps is a tool that should only be used for entertainment purposes only and should not be used to make territorial, political or military decisions".
Most of the island lies within the undisputed territory of Costa Rica. However, a territorial dispute dating back to 1850 between the two Central American nations resurfaced in 2010 over the northwestern section of the island, about 3 km in length, alternatively identified by the Nicaraguans as either Isla Portillos or Harbour Head Island. Nicaragua had shown this part to be Costa Rican on all of their own maps for the previous half-century, based on the 1858 Cañas–Jerez Treaty which established Nicaraguan ownership of the San Juan River, but gave Costa Rica the right to navigation for commercial purposes. Nicaragua's claim beginning in 2010 and contradicting the map attached to the 1897 Alexander Award, is that a small intermittent channel about 3 km south of the northern tip of the island is the channel which the boundary should follow, not the main San Juan River channel. In Nov. 2010 Nicaraguan military forces clearcut forest along this channel and deepened it.
In March 2011, The International Court of Justice provisionally ruled that Costa Rica and Nicaragua both must refrain from sending or maintaining civilians, security forces or police in this disputed border area, but that Costa Rica was allowed to send civilian teams concerned with environmental matters. Dredging by Nicaragua within the San Juan River itself was allowed to continue since Nicaragua has sovereignty over the river proper.