During President George W. Bush's second term, Congress authorized $1.2 billion to build several hundred miles of double-layer fencing, but the government faced myriad obstacles. Private landowners objected to buyout offers. There were environmental concerns and lawsuits.

Constructing the wall, now a signature applause line at Trumpcampaign rallies, is a complicated endeavor, fraught with difficulties. Numerous bureaucratic, diplomatic, environmental, monetary, and logistical hurdles must be overcome.

Trump will have to navigate not only the treaty maintained by the International Boundary and Water Commission but also various environmental regulations that protect some stretches of border and restrict what kind of structures can be built and where. The contracting notices of March 17 say the Trump administration wants the wall dug at least 6 feet into the ground. Along parts of the border in California, environmentally sensitive sand dunes required that a "floating fence" was built to allow the natural movement of the sand.