By Tahiat Mahboob  

After announcing that he would run for the United States presidency in June 2015, Donald Trump debuted his plan to build a wall on the southern border at a campaign rally in Phoenix, Arizona, a month later. "I will build a great great wall on our southern border and I'll have Mexico pay for that wall," he said.

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Trump’s War on the Border

At the time of this promise, parts of the U.S.-Mexico border already had a fence to prevent border crossings. Here’s what you need to know about the existing barrier:

1. Of the 3,145 kilometers U.S.-Mexico border that runs through California, Arizona, New Mexico, and Texas, more than 1100 kilometres already has fencing, known as tactical infrastructure. This interactive map by USA Today explores the different types of barriers currently in place.

2. There are three main categories of fencing: primary pedestrian fencing, secondary fencing behind the pedestrian fencing, to additionally deter border crossings by foot, and vehicle fencing.

3. The U.S. Border Patrol first began erecting a barrier known as the “primary fence” directly on the border in 1990 to deter illegal entries and drug smuggling in its San Diego sector.

4. In 2006, then president George W. Bush, signed the The Secure Fence Act of 2006. This act called for the creation of more than 1100 kilometres of fencing. Barack Obama and Hillary Clinton who were both senators at the time voted for this project. The act passed in the Senate by a vote of 80 to 19.

5. The Secure Fence Act of 2006 also authorized additional patrol measures that include the use of more vehicle barriers, checkpoints and lighting, and advanced technology such as satellites and unmanned aerial vehicles. The U.S. Customs and Border Patrol surveillance portfolio includes cameras, underground sensors, aircrafts, drones, mobile surveillance and boats to monitor and deter border crossings.

6. According to images released by the U.S. Customs and Border Protection in 2011, the type of barrier varies depending on terrain and the population of the area. The different types of border material include: bollard, chain link, concrete, landing mat, post and rail, sheet piling, steel beams, and wire mesh.

7. The structure of the barrier also varies. California, Arizona, New Mexico and West Texas, and Southeast Texas each utilize four different types of structures, sometimes combining materials such as concrete and bollard to erect a barrier.

8. According to the U.S. Government Accountability Office (GAO), the border fencing constructed by the end of 2007 fiscal year cost approximately $2.8 million a mile. It was mostly built by the U.S. Army Corps of Engineers and the National Guard. The border fencing constructed in fiscal year 2008 cost approximately $5.1 million a mile by private constructors. In total the U.S. has already spent $2.4 billion on the barrier constructed under the Secure Fence Act.

9. Sections of the border in Texas go through privately owned property. Homeland Security asked property owners to sign away their rights to their land for the fence. When property owners refused, the U.S. government used the power of eminent domain – the power of the government to expropriate private land – and condemned their property.

10. Stretches of the border running along Texas also contain the Rio Grande river. A 1970 treaty between the U.S. and Mexico prohibits building anything that obstructs the normal flow of the river. To honour this treaty, some sections of the existing border fence were built significantly inland, sandwiching some U.S. properties between the Rio Grande and the fencing.