Illegal invasion is nothing more than stretching the meaning or relating to a military invasion.
Military invasion is clearly defined as enemy troops
You can use invasion like invasion of privacy does really mean the same as military invasion ,
Summer or winter resorts have an invasion of people when a bunch of people show up for an event or the watch the leaves turn brown.
Applying illegal invasion and them implying that it is a military invasion is just padding an argument using unrelated terms.
If us want to say that it is illegal that they want to come to the US then say it.
If the military see someone crossing the border all they need to do is keep a tabs on their position and call in the border patrol
I would imagine standing there watching them would probably be deterrent enough
There is a border patrol and that is there job, if there are not enough people then hire more people.