The Organization of American States (OAS) heard arguments this week from Costa Rican Foreign Minister René Castro on the Nicaraguan military’s alleged “incursion” on to Costa Rican soil. And now, with tensions continuing to heat up, OAS Secretary General José Miguel Insulza arrives in San José tonight for talks with Minister of Foreign Relations René Castro and President Laura Chinchilla. He will continue to Managua on Saturday morning to meet with President Daniel Ortega.

The dispute is over Isla Calero on the San Juan River. On Monday, Costa Rica’s security ministry reported seeing a Nicaraguan flag, five soldiers and tents on Isla Calero, an island Costa Rica claims as its own. Costa Rica says this is a violation of its national sovereignty.

Costa Rica and Nicaragua have been trading barbs over who is violating whose sovereignty and who is even sending troops on whose turf since Nicaragua’s San Juan River dredging project began last month.

Costa Rica first decried alleged environmental foul play by Nicaragua, claiming it dumped sediment from the river dredge and then cut down Costa Rican trees, all on its side of the San Juan. Costa Rican Security Minister José María Tijerino also accused Nicaragua of secretly planning to carve a canal across its territory. Costa Rica deployed well-armed and camouflaged police to the border, while the authorities sought communication through diplomatic channels.

Just when it seemed Costa Rica’s shouts were falling on deaf ears, Nicaragua’s foreign ministry lashed back in a strong-worded written reply to its Costa Rican counterpart, demanding that Costa Rica stop violating its sovereignty. Nicaragua protested repeated incursions by “Costa Rican armed forces,” and the Nicaraguan Foreign Ministry then issued a formal letter of protest.

Costa Rica’s government almost seemed delighted in responding to those remarks, reminding its neighbor that Costa Rica had in fact disbanded its army more than 60 years ago. The media was so anxious to write up President Chinchilla’s reply that the president accused La Nación, a leading Costa Rican newspaper, of misquoting her in a headline as saying “Nicaragua forgot where its border is.” She claimed what she really said was that Nicaragua forgot who its neighbor was.

Sound bites have been abundant during this dispute, which one columnist called a tragicomedy. But the most waves in Costa Rica spread from quotes by Edén Pastora, a former Sandinista revolutionary hero who Nicaraguan President Ortega put in charge of the San Juan dredging project. Pastora said on national television that the land in question is tierra de nadie, or no man’s land. Costa Ricans are still fired up by this. How dare he disrespect a country that took him in when he was a guerrilla fighter, many people are saying.

It should be no surprise that the San Juan River is flowing at the heart of another bilateral dispute between the countries. The river, which belongs to Nicaragua, is more than a natural border between the countries. It’s been a point of contention for over a century. Some believed the troubles had been settled during a July 2009 World Court ruling, which seemed to be a happy ending to an old row over river navigation rights. But a geographer quoted this week in La Nación dusted off an accord dating back to 1868, the Cañas-Jerez Treaty, saying it indicates that the little island with a Nicaraguan flag really belongs to Costa Rica.

Costa Rica has sent more police to the area, but Tijerino, the security chief, said he will avoid deploying them onto the disputed island itself for fear of clashing with Nicaraguan army.

“We’re going to avoid as much as possible a confrontation, which would only aggravate the situation for who knows how long,” Tijeron said.

*Alex Leff is a contributing blogger to americasquarterly.org based in San José, Costa Rica, and is the online editor for The Tico Times, Central America's leading English-language newspaper.