The Posse Comitatus Act is often cited as the reason troops cannot be used to protect the border. As has been experienced, the perception of the meaning of certain laws sometimes change over time, due to activist judges, who legislate from the bench, progressive politicians, who believe the Constitution (and the law) should be a “living document” whose interpretation should change, according to the whim of the times. A biased, propagandistic media, aligned with progressives and neo-Marxists, helps to brainwash the public. This diabolical trend really needs to be reversed and laws enforced according to their original intent.
The Posse Comitatus Act, passed in 1878. It was updated in the 1950’s to include the Air Force. Enacted as part of a deal at the end of Reconstruction, it was intended to prevent the military from enforcing martial law on the general public, as it did in the South during Reconstruction.
18 U.S. Code § 1385 – Use of Army and Air Force as posse comitatus
Whoever, except in cases and under circumstances expressly authorized by the Constitution or Act of Congress, willfully uses any part of the Army or the Air Force as a posse comitatus or otherwise to execute the laws shall be fined under this title or imprisoned not more than two years, or both.
The Posse Comitatus act contains an exception, “except in cases and other circumstances authorized by the Constitution of act of Congress”.
In the U.S. Constitution, the United States is required to protect each of the states against an “invasion”. An ordinary law cannot override the Constitution. So, an exception in a law is not necessary to carry out a constitutional mandate.
U.S. Constitution, Article IV, Section 4.
The United States shall guarantee to every state in this union a republican form of government, and shall protect each of them against invasion; and on application of the legislature, or of the executive (when the legislature cannot be convened) against domestic violence.
It does not say “armed invasion”, “violent invasion” or even “hostile invasion”, but just “invasion”. There are probably 500 thousand to one million foreigners crossing the border illegally every year. What is that, if not an invasion? At times it is an armed, hostile and violent invasion, too. It seems appropriate for the President to use the Armed Forces as required to stop this invasion, not just one caravan, but to help shut down the mass illegal border crossing that has been going on for decades unabated.
The image on the right is the cover of a book about one soldier’s experiences, when he was assigned to guard the U.S. border during the 1910’s, a decade before the Border Patrol existed.
Below is another photo from the same book of a squad of troops in 1916 in full combat gear, guarding the border, eight years before the Border Patrol existed. In both images the Rio Grande is in the background, that is the U.S. border with Mexico.
It seems absurd to think that the Constitution would not allow the President to protect the border against invasion by hundreds of thousands of foreign, illegal border crossers every year. That was not the intention of the framers of the Constitution and it obviously was not the intention of those who passed the Posse Comitatus Act, either.
At this time mainly observers are needed, who are directly on the border and are armed so that they can defend themselves, if necessary. Military observers can report any activity to the Border Patrol, who can actually make the arrests.
While waiting for a more effective border barrier to be built, a few thousand effectively-deployed troops would make decisive difference, virtually shutting down illegal border crossing.
Below is a Texas National Guardsman observing for the Border Patrol on the Rio Grande. The military can play the same role and is directly under the control of the President. 
Texas would allow their guardsmen to be armed and directly on the border acting as direct observers, California would not. States could put other limitations on the guard, making them much less effective.
Since the national guard consists of part-time soldiers with families and jobs it is a greater hardship to deploy them for a long time period. Military support of the Border Patrol needs to be uniform and well-planned to be effective. They need to be well-coordinated with the Border Patrol and deployed directly on the border with arms, until the southern border is really under control.