The United States shall guarantee to every State in this Union a Republican Form of Government, and shall protect each of them against Invasion
US Const., Art IV, Sec. 4.
If you want clear federal gov’t illegality from the Supreme Law of the Land, the Constitution, call them “INVADERS” which term the Left avoids.
My guess (we don’t have investigative reporting much anymore, so I don’t know) is that these NGOs are training “migrants” to lie on applications so they can claim to be escaping persecution (asylum claim). That would make them legally migrants until their cases are heard. So I suppose invader is more accurate than illegal alien. The illegals would be all those sneaking across.