The New Republic has an article listing 6 reasons why the border fence being created between the US and Mexico won’t work.

Among the reasons:

It doesn’t work. Most experts say that physical fencing would not drastically decrease the number of illegal immigrants entering the country. The Pew Hispanic Center estimates that as much as 40-50 percent of the U.S.’s unauthorized migrant population entered the country through legal ports of entry, either with nonimmigrant visas that subsequently expired (known as “overstayers”) or by using a Border Crossing Card that allows for short visits to the border region. A fence spanning the border would not prevent their entry to the country.