The wall would go a LONG way to making the Mexicans stay and fight, rather than running across the border into the “arms” of this country. It would be far too easy to simply run across an un-walled border. Our border people would be overwhelmed at the mad rush! That is simple logic.
A wall helps, but obstacles must be covered by observation and fire. If Mexico descends into revolution, troops will have to build defenses in developed areas and effectively screen the undeveloped border in order to deter and defeat attempts to cross.