Venezuela thinks a large part of Guyana doesn’t actually belong to the Guyanese. Tensions over the issue are mounting to the point there could be an official armed conflict between two South American countries for the first time in three decades.

The tensions center on a disputed part of Guyana called Essequibo. Venezuela, a former Spanish colony, said Essequibo was stolen in 1899 and improperly given to Guyana — a former British colony — by an international arbitration tribunal.

The disputed area is about the size of Greece and makes up about two-thirds of Guyana’s total land mass. Essequibo is a geographically diverse region full of mountains, valleys, rivers and dense jungle. It’s also rich in natural resources like gold and oil.

The dispute over Essequibo mostly died down over the last century or so, but it renewed in 2015 when ExxonMobil discovered a massive oil deposit off the coast of the Essequibo region. The deposit is large enough to account for around 1% of total global output once extraction begins.

Venezuelan President Nicolás Maduro just held a referendum in Venezuela on which country should control Essequibo. Venezuelan elections aren’t known for integrity, and — even though very few voters were seen on election day at polling locations — Maduro’s regime said more than 10 million votes were cast in favor of Venezuela annexing Essequibo. That could mean ignoring the United Nations and the International Court of Justice, both of which officially recognize Essequibo as Guyana’s sovereign territory. No one living in the disputed territory was given a chance to cast a ballot.

While politicians and pundits pontificate on the likelihood of a Venezuelan invasion into Guyana, military assets in the region are preparing for the possibility.

Brazil’s military said it is intensifying defensive actions along its northern border while monitoring the dispute. Venezuelan forces are also being observed along the border with Guyana, performing their own military drills.

At 123,000 active-duty soldiers, Venezuela’s military is one of Latin America’s largest and much larger than Guyana’s standing army, which totals around 3,000. Venezuela’s military is armed with modern weaponry like armored vehicles, tanks, fighter jets and naval vessels including patrol boats, submarines and landing craft.

Most of the border between Guyana and Venezuela is mountainous. The thick rainforest and waterways in the Essequibo region would make large-scale troop movements difficult. In this type of environment, smaller squads trained in jungle warfare would make more sense than trying to send in a bunch of tanks that would likely get stuck and make easy targets.

Also, Essequibo is a rainforest. So, the jungle’s canopy and unpredictable weather would no doubt impact aerial operations as well. If an invasion does come, an amphibious assault may make the most sense.

Maduro and Venezuela still have many factors to consider before launching an invasion. The country is trying to regain international favor and put an end to some economic sanctions. Both the U.S. and China are financially invested in the oil fields Venezuela wants to take over. Furthermore, Guyana has military agreements in place with the United States, and the two countries have held regular drills together for the past 20 years or so.

The Associated Press contributed to this report.