U S. Constitution, Section 10; the actual text of the pertinent portion:
“No State shall, without the Consent of Congress, lay any duty of Tonnage, keep Troops, or Ships of War in time of Peace, enter into any Agreement or Compact with another State, or with a foreign Power, or engage in War, unless actually invaded, or in such imminent Danger as will not admit of delay.”
So I guess legality here all comes down to how one interprets “invaded”.
On an average day, a force about 9 times the size of Santa Anna’s army that attacked the Alamo crosses over into Texas. In the last year alone a force that roughly doubles the size of the Russian, Saudi, and Mexican Armies combined has crossed the border.
The vast majority are military aged men.
So yeah, that fits the definition of an “invasion”.
RE: invade
Merriam Webster
in·vade
/inˈvād/
verb
(of an armed force or its commander) enter (a country or region) so as to subjugate or occupy it.
“it was all part of a grander French plan to invade Ireland”
Similar:
occupy
conquer
capture
seize
take (over)
annex
win
gain
secure
march into
overrun
overwhelm
storm
descend on
swoop on
swarm over
surge over
make inroads on
attack
assail
assault
raid
plunder
maraud