The first is in Section 102 of the Illegal Immigration Reform and Immigrant Responsibility Act (IIRIRA), a law which Congress passed in 1996 and has subsequently amended several times. The law, codified here, gives DHS a clear mandate to construct reinforced fencing along at least 700 miles of the 1,933-mile land border with Mexico.

Specifically, IIRIRA requires barriers to be built in areas of “high illegal entry” to the United States, and grants the Secretary of Homeland Security the ability to waive “all legal requirements” that may impede this construction.

According to one expert analysis, “nothing in current statute would appear to bar DHS from installing hundreds of miles of additional physical barriers … even beyond the 700 miles required by law.”

So why hasn’t DHS simply done this already? It’s a matter of prioritization — and of funding.