Trump's border wall with Mexico faces all kinds of obstacles
Trump's first budget proposal to Congress, a preliminary draft that was light on details, asked lawmakers for a $2.6 billion down payment for the wall.
Congressional Republicans have estimated a more moderate price tag of $12 billion to $15 billion.
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.
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.
The Trump administration appears to be preparing for the legal fight and included a request for more lawyers to handle such cases in its budget proposal.