for centuries, societies have erected walls and fences to separate themselves from their neighbors, from the great wall of china through the berlin wall right up to the barrier that today divides israel from the palestinians on the west bank.









the united states has debated putting up security barriers of its own along the southwest border and has spent billions of dollars in recent years fencing one-third of it.









now, donald trump is proposing to go even further, vowing to build a massive, impenetrable wall along the u.s.-mexico frontier to keep out illegal mexican migrants. 









<u+201c>building a wall is easy, and it can be done inexpensively,<u+201d> the republican presidential candidate said in an interview. <u+201c>it<u+2019>s not even a difficult project if you know what you<u+2019>re doing.<u+2019><u+2019>









the wall has become the signature proposal of trump<u+2019>s campaign, which has stirred widespread controversy over its focus on illegal immigration and his comments about immigrants.









any wall-building effort would cost billions of dollars and encounter a variety of obstacles, according to experts, documents and federal officials, including some of the same difficulties that bedeviled the federal government as it spent more than $7 billion on border fencing. the hurdles include environmental and engineering problems; fights with ranchers and others who don<u+2019>t want to give up their land; and the huge topographical challenges of the border, which runs through remote desert in arizona to rugged mountains in new mexico and, for two-thirds of its length, along rivers.









<u+201c>it<u+2019>s extremely challenging to put a brick-and-mortar wall along the southwest border for any number of reasons,<u+201d> said richard stana, who wrote multiple reports on border security for the nonpartisan government accountability office before retiring in 2011. <u+201c>it seems very simplistic.<u+201d>









if such a barrier could be erected, experts and government officials agreed that making it impenetrable would be virtually impossible, as is completely securing the entire 1,954-mile border. the department of homeland security is already spending millions of dollars a year to maintain existing fences and to repair breaches, according to government reports and officials, while drug traffickers and smugglers are increasingly using tunnels to pass underneath.









while a wall along much of the border might theoretically be possible, said thad bingel, a former senior u.s. customs and border protection official, <u+201c>is it desirable? at what cost, and what do you give up to pay for that?<u+2019><u+2019>









bingel <u+2014> who was involved in border fence-building during the george w. bush administration and is now a partner at command consulting group in washington <u+2014> added: <u+201c>every wall can be circumvented. people can go under it, they can go over it. .<u+2009>.<u+2009>. no one should go into this with the idea that if you just build the right kind of wall, no one will get through.<u+2019><u+2019>









trump disputed that, saying that a wall <u+201c>would be very effective<u+201d> in deterring illegal migrants and that seismic and other equipment could detect and stop any underground tunnels. <u+201c>a wall is better than fencing, and it<u+2019>s much more powerful,<u+201d> he said. <u+201c>it<u+2019>s more secure. it<u+2019>s taller.<u+201d>









the veteran builder acknowledged that environmental impact studies would be difficult but said he is the one person who can rise to the challenge. <u+201c>i<u+2019>m considered a great builder, by everybody,<u+201d> he said, adding that cost is irrelevant because he would force mexico to pay for the structure. asked whether that was realistic, trump said: <u+201c>it<u+2019>s realistic if you know something about the art of negotiating. if you have a bunch of clowns negotiating, it<u+2019>s not realistic.<u+201d>









trump has emerged as a leading gop candidate partly because of his strong statements about immigration, which have included describing mexicans entering the country illegally as <u+201c>rapists<u+201d> and <u+201c>murderers.<u+201d> he has suggested at times that his proposed wall would be extensive and would cover nearly the entire border, but said in the interview: <u+201c>you don<u+2019>t have to build it in every location. there would be some locations where you would have guards, where you don<u+2019>t need it because the topography acts as its own wall, whether that<u+2019>s water or very rough terrain.<u+201d>









[univision cuts ties with trump over comments about mexican immigrants]









the concept of a wall or fence along virtually the entire border has bubbled up occasionally in the nation<u+2019>s immigration debate, with some republicans supporting the idea. today, there are more than 45 such walls and border fences worldwide, perhaps most prominently israel<u+2019>s west bank barrier.









while israeli officials say it has reduced attacks, security specialists say that barrier, slated to be more than 400 miles long when finished, is not comparable to what would be required along the far more extensive u.s. southwest border. the israelis, they add, supplement the physical concrete barrier with a mix of border police and technology, much as the department of homeland security does in the united states.









the u.s. government began building border fencing near san diego in 1990. as dhs cracked down on illegal immigration after the sept. 11, 2001, attacks, president george w. bush dramatically expanded the effort. spending on border fencing and related infrastructure such as lighting shot up from $298 million in 2006 to $1.5 billion the following year, according to the nonpartisan congressional research service.









overall, more than $7 billion has been spent to build what is now almost 653 miles of southwest border fencing <u+2014> costing nearly $5 million per mile in some spots <u+2014> nearly half in arizona.









the costs could rise substantially if extensive new fencing was built, since it would be in increasingly remote regions without roads and in mountainous terrain, said marc rosenblum, deputy director of the u.s. immigration policy program at the migration policy institute. adding even more to the expense, he said, would be acquiring private land near the border and maintaining existing fencing.









trump<u+2019>s wall would probably cost far more than fencing, stana said, given the greater needs for construction materials and labor.









while current and former dhs officials say the fencing has been effective in deterring illegal immigration, they say it is only one part of a broader border strategy that includes expanded sensors, drones and other technology, along with growing numbers of border patrol officers.









<u+201c>our southern border is a mixture of winding river, desert and mountains. simply building more fences is not the answer,<u+201d> dhs secretary jeh johnson said in an october speech.









the government<u+2019>s difficulties in erecting fences highlight the challenges of building a wall, experts said. the fencing mandated by congress in 2006 was beset by delays, surging construction costs and disputes with private property owners, mostly in texas, dhs officials have said. the biggest failure was the virtual fence, a bush administration effort to cover the border with a high-tech surveillance system.









<u+201c>it<u+2019>s a huge effort to construct anything at the border,<u+201d> said one dhs official, who has worked in republican and democratic administrations and spoke on the condition of anonymity because trump<u+2019>s plan is part of a political campaign. <u+201c>you have lots of requirements to do construction: the environmental piece, engineering assessments. and a private landowner might not want fencing.<u+201d>









wayne cornelius, director of the mexican migration field research program at the university of california at san diego, called trump<u+2019>s proposal <u+201c>ludicrous. .<u+2009>.<u+2009>. any physical barrier can be tunneled under or climbed over or gotten around. there will always be gaps, and smugglers and migrants will seek out those gaps and go through.<u+201d>









robert costa and alice crites contributed to this story.
trump says building a u.s.-mexico wall is <u+2018>easy.<u+2019> but is it really?
noauthor