iStock/Thinkstock(WASHINGTON) — Defense Secretary James Mattis defended the deployment of thousands of active duty troops to the border with Mexico Wednesday, calling it “a moral and ethical mission to support our border patrolmen.”While visiting with some of the soldiers deployed to the southern U.S. border, Mattis said “border security is part of national security” and stressed that the military mission is strictly to support U.S. Customs and Border Protection as it prepares for the arrival of four migrant caravans in the coming weeks.“We determined the missions as absolutely legal and this was also reviewed by Department of Justice lawyers, it’s obviously a moral and ethical mission to support our border patrolmen,” Mattis told a small group of reporters traveling with him to Texas.On Wednesday, Mattis visited Camp Donna, a large, newly-constructed camp outside of McAllen, Texas that was built to house 1,000 soldiers and their equipment.He was accompanied by Homeland Security Secretary Kirstjen Nielsen, who oversees Customs and Border Protection, and Gen. Terrence O’Shaughnessy, head of U.S. Northern Command.As of Wednesday, there were 5,900 troops deployed on the border. More than 7,000 active duty troops are expected to be fully deployed in Arizona, California and Texas ahead of the arrival of the migrant caravans.“The eyes of the world, and certainly all of the Americans, are on you,” Mattis told soldiers during his tour of Camp Donna.Some military analysts have speculated that the deployment could end up costing as much as $200 million, but Mattis told reporters traveling with him that he was not able to provide a preliminary cost estimate because the information was still flowing into the Pentagon.“So we can estimate costs all we want,” said Mattis. “I prefer to give you real costs.”Four migrant caravans were making their way through Mexico to the U.S.Several hundred migrants from the first caravan arrived in Tijuana on Tuesday where they are expected to rest and receive legal advice before approaching the border crossing at San Ysidro to request asylum.Last week President Donald Trump signed an executive order that only allowed asylum requests to be made at formal ports of entry, a change of existing law that allows asylum requests anywhere on the border.Migrants from a caravan that left Mexico City earlier this week and are now headed to the border crossings near Tijuana, 1,500 miles to the northwest. That was a change from the expected route to McAllen and Brownsville, Texas, a much shorter distance.Mattis acknowledged the location of the caravans arrival was a “dynamic situation” but said “we can move the troops back and forth.”“Right now the only thing we’ve been asked for is to put in obstacles, provide transportation and to provide housing,” said Mattis, reiterating that the soldiers’ mission along the border is one of support.“I do not anticipate military personnel coming into direct contact with migrants,” said Mattis.“I’m a hundred percent confident we have the number of troops at each of those ports of entry to complete what we’ve been asked to do prior to the arrival of the large caravans.”Additionally, the only military personnel who will be armed are the military police providing security to the military support units.“The service members there in an engineering capacity who are building barbed, wire and barriers are not armed,” said Mattis.Military police watching over these engineers, he said is part of normal force protection.The military deployment became a politically charged issue in the lead-up to the midterm elections with Democrats questioning whether it was necessary and if Trump was using the military for political gain.While campaigning, Trump regularly referred to the U.S. military mission to help deal with the migrant caravans which he labeled as an “invasion.” But since the election, the president has not mentioned the border mission publicly or on social media. He has also not used the term “invasion.”Last week, Mattis instructed that the name “Operation Faithful Patriot” be dropped and instead refer to the deployment as a border support mission. He explained he did not want to risk misinterpretation by using military terminology to describe a mission supporting another agency.“I do not want to put this mission in some arcane military terms if what we are doing is laying wire,” said Mattis. “I want to talk to the American people because this is a highly politically visible issue and I want you to tell them what you’re doing.”Copyright © 2018, ABC Radio. All rights reserved.