You read an article that said 40% of illegals come by air. That article is not accurate anymore. In fact most illegal immigrants in the US are people who overstay their visas and like Canada are most likely to be from China or India which would be people who come from air yes but as well there are huge movements of people by container ships, or motor vehicles trains and trucks as well. In fact illegals by air only become illegal usually after overstaying their visa. If you are talking smuggled illegals then its containers on ships, trains and trucks or people hidden in compartments of trucks and cars or by tunnel. Surface travel is done but its not the major source and people under estimate the ability of US border patrols to pick up on movements due to satellite technology now. They may not catch them until they are across the border but they are catching them.
Illegals from Mexico but who are not necessarily Mexicamn are going to come in, hidden in cars, trucks, containers on ships, trains, trucks and by tunnel. In fact Mexican citizen migration levels are in decline as less Mexicans are having children, and there are more jobs now in Mexico. If anything the people coming across the Mexican border could be Central or South American or West Indian if they haven't tried to get in my Florida or other Gulf coast or Atlantic coast boats ranging in size from rafts to large container ships and trawlers.
Whether walls will be effective I agree with you is questionable. Its easy to dig under them. It does not stop tunnels. I mean the volume of people climbing over walls where they do exist is not the significant source of illegals and never was. Wall building is a political pandering exercise. It turns the intangible into tangible it provides an easy eye reference to a very complex series of movements of people. Its based on the simplistic idea that people walk into the US easily. They have the US mixed up with Canada where Trudeau has enabled illegals to flow into Canada by simply walking in collapsing our screening system and making a farse of any immigration or refugee laws we had.
The fact is illegal US immigrants came in with full knowledge of state governments for many years as they fully understood they did the sweat jobs (farming, factory, cotton picking) Americans were not going to do. Mexicans now are not as hard pressed to come to the US as free trade with the US made it easier for them to build cars in Mexico and set up cattle, pig and farm operations that export into the US not to mention the garment business. Mexicans went to the US for jobs that now exist in Mexico because of free trade. Ironically Trump wants to end free trade with Mexico the very thing that has curtailed their movement into the US. Trump wants to end free trade with Mexico and Canada to try end Canadian and Mexican trade advantages. He wants it both ways. No Mexican illegals coming for work, but when they stay behind and have work, he wants that work ended as its too competitive with US work.
That said the majority of Americans want a wall, and its their right to build one. Its a sovereign issue. What Yanks do within their borders is their right. They want to build walls, its their right.
I don't think Mexicans were a true criminal or terrorist threat to the US in fact they provided a cheap source of under the table labour. Now that they under free trade can stay home I am not sure what Trump is thinking singling out Mexicans and trying to end their jobs by ransacking NAFTA. If he puts them out of work they will be forced to head to the US again. Some argue Trump is building the walls knowing they will be useless and his real agenda is to end NAFTA to get the cheap Mexican labour out of Mexico annd back into the US as illegals so they can be paid again under the table under minimum wage knowing they will just dig tunnels. Mexicans right now are no more than 28% of illegals in the US and the majority of Mexican illegals have been in the US more than 10 years or more.
Today's illegal in the US is most likely a Chinese or Indian student overstaying their student visa.
All that said the walls were not something Trump has created. In fact Obama started the building of them during his first term in office, not Trump and through-out his 8 year tenure Obama approved funding for their construction. Trump is simply continuing what was already started.
Obama wanted to grant amnesty to illegals, Trump does not. That's the real difference in their approach to illegals. Even the screening of Muslims or people from certain nations was going on in the Obama era no different than it was back when Regan, Clinton and Bush reigns.