In taking his first step towards building a US-Mexico border wall, US president Donald Trump begins an attack on a vanishing issue. Today, Trump signed an executive order to direct federal resources to build a wall intended to staunch the flow of Mexican immigrants entering the US illegally.

In reality, undocumented immigration from Mexico to the US is less of an issue that it has been in over 40 years. And more recently, due to a growing Mexican economy, the number of people attempting to cross the US-Mexico border fell precipitously over the last 15 years. Since 2007, the total number of Mexican immigrants in the US has actually been on the decline.