This is one of the barriers at the US-Mexico border crossing in Brownsville. REUTERS/Shannon Stapleton Perhaps no debate in the US has been as heated in the past decade as the one over control of the US-Mexico border. At the center of the argument are cities like Brownsville, Texas, which is so close to nearby Mexican cities that local residents can see them from their backyards.

Unlike many other towns in Texas, Brownsville and Mexico are divided by a natural border — the Rio Grande. While many might think that this would make building a border fence simpler, it has had the opposite effect.

A decades-old treaty with Mexico prohibits building in the Rio Grande floodplain, forcing the US government to build its border fence more than a mile north of the river, effectively cutting into thousands of acres of property owned by Americans.

A house in front of the border fence at the US-Mexico border in Brownsville:

A house is seen in front of the border fence at the United States-Mexico border in Brownsville, Texas August 6, 2014. REUTERS/Shannon Stapleton

A U.S. flag blows in the wind in the backyard of a home facing the border fence at the United States-Mexico border outside of Brownsville, Texas, August 5, 2014. REUTERS/Shannon Stapleton

Depending on where you live, you could end up on the "Mexican" side of the border, even though you are still in the US. Many Texans had their property split in half by the fence or, worse, seized by the government.

Crystal Ibarra, 23, lives in a house facing the border fence, which she can see from her backyard.

Crystal Ibarra, 23, reaches for her laundry in her backyard, which faces the border fence at the United States-Mexico border outside of Brownsville, Texas, August 5, 2014. REUTERS/Shannon Stapleton

Even with the border fence, the divide between Mexico and the US can be hazy. Here is a sign in Spanish on the US side of the border in Brownsville.

A sign in Spanish that says "Entrance" is seen at a field in front of the border fence at the United States-Mexico border outside of Brownsville, Texas, August 4, 2014. REUTERS/Shannon Stapleton

The border fence is 18 feet high and made of steel and concrete; it cost American taxpayers $6.5 million per mile. In the Rio Grande Valley there are 54 miles of border fence broken up into 18 sections.

Here's what the fence looks like up close:

The moon is seen high above the border fence at the United States-Mexico border outside of Brownsville, Texas, August 5, 2014. REUTERS/Shannon Stapleton The border fence stands at the United States-Mexico border along the Rio Grande river in Brownsville, Texas, August 5, 2014. REUTERS/Shannon Stapleton

The border fence is anything but seamless. There are mile-long gaps in the fence, which the government says it plans to close with 15-foot gates, with keypads and codes for each landowner.

The border fence has been criticized as an ineffective deterrent that jeopardizes the health of not only those seeking illegal entry but also entire animal habitats. The border fence prevents numerous species from reaching watering holes and following migration patterns.

Proponents say it funnels would-be illegal immigrants toward the few openings in the fence, like this entry point at the border crossing in Brownsville.