The Venezuelan Navy illegally entered the waters of Guyana this weekend and forced a ship contracted by Exxon Mobil to conduct oil research in the area to vacate, claiming that Guyana’s permission to explore its sovereign territory was not enough for the ship to be legally present in the water.

The incident, which Guyanese authorities angrily denounced and vowed to bring to the attention of the United Nations, reignites a feud Venezuelan socialist dictator Nicolás Maduro began with the neighboring country in 2015, claiming as much as two-thirds of Guyana itself belonged to Venezuela. Guyana has repeatedly noted that Venezuela signed an agreement in 1899 on the territory in question and no disputes remain as to who owns that land.

Exxon Mobil made its first of ten major oil discoveries in Guyana in 2015, triggering Maduro’s claims to the territory. Despite being an OPEC member nation and home to one of the world’s largest known oil reserves, Venezuela has been forced to import hundreds of thousands of gallons of refined oil because the socialist state nationalized the nation’s major oil corporations and has replaced experts at Petróleos de Venezuela (PDVSA), the state-run oil company, with Maduro cronies of limited experience in the oil industry.

Reuters identified the vessel intercepted as the Ramform Tethys of Petroleum Geo-Services (PGS), a contractor working on behalf of Exxon Mobil researching seismic activity in the region. Only Venezuela identifies the waters where the incident occurred on Saturday as exclusively Venezuelan; they are Guyanese waters by international law.

The Venezuelan Foreign Ministry nonetheless justified its actions in a statement this weekend, explaining that a Venezuelan navy ship “engaging in its customary patrols in the Venezuelan waters of the Atlantic … identified the unauthorized presence in Venezuelan territorial waters of two seismic exploration ships.” The statement described the Guyanese waters as “within the indisputable sovereignty of Venezuela.”

“Before this flagrant violation of sovereignty, [the Navy] applied proper international protocol for these incidents and safeguarded the sovereignty of the nation with strict adherence to international agreements and treaties,” the Venezuelan government statement continued.

According to Reuters, the Navy approached the exploration ships and questioned their presence in the area. The crew onboard explained that they were in sovereign Guyanese waters and had the permission of the government of Guyana to be there, which the Venezuelan soldiers claimed was not enough.

“It is important to note that … those onboard argued that they had a permit from the Republic of Guyana to operate in this maritime space,” the Venezuelan statement read. “They were informed that this country does not have sovereignty in any of the maritime territory of the Orinoco [River] Delta, which led them to navigate away.”

In its own statement released Sunday, the government of Guyana expressed outrage at the Venezuelan government invading its territory and said it would bring a complaint before the United Nations.

The incident “shows the true threat to economic development of the country posed by its western neighbor,” accusing Venezuela of “violat[ing] the sovereignty and territorial integrity of our country.”

“Guyana rejects this illegal, aggressive and hostile act,” the statement asserted.

The U.S. State Department weighed in Sunday in defense of Exxon Mobil and of the sovereignty of Guyana.

“We underscore that Guyana has the sovereign right to explore and exploit resources in its Exclusive Economic Zone. We call on Venezuela to respect international law and the rights of its neighbors,” the short statement read in part.

The incident is the first in years to reignite tensions over a territory called the Essequibo, a sparsely populated, resource-rich region that constitutes two-thirds of the entire territory of the state of Guyana. Maduro announced a campaign to claim the Essequibo in 2015, shortly after Exxon Mobil announced its first discovery of vast oil resources in the waters off the coast of the region. Maduro established an exclusive “maritime zone” that year over Guyanese waters, a declaration of no international legal value, and threatened Exxon Mobil and other companies to stay out of the region unless they explicitly received permission from Caracas.

Guyana’s government responded at the time with confusion and outrage, noting that the issue had been resolved via international agreement in 1899 without controversy. The Guyanese government’s total rejection of Venezuela’s claims did not stop the Navy from attempting to enforce the “maritime zone,” leading to stern calls from Guyana for Venezuela to exit the region. In July 2015, Venezuela recalled its ambassador to Guyana. Maduro claimed “it takes a lot of patience to process, digest and not vomit when one reads and hears the statements against Venezuela … by the current president (of Guyana)” that his claims to the Essequibo had no international legal basis.

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