There is a Reason for Posse Comitatus

and this is it.

Speaking with reporters on Wednesday, Mattis confirmed that he had been given the additional authority, but would await a request from the Department of Homeland Security (DHS) before deciding how to use it.

“I have the authority to do more, now we’ll see what she asks me for,” Mattis said, referring to DHS Secretary Kirstjen Nielsen. He said he had no immediate intention of changing the mission.

Mattis said, depending on the DHS request, that troops may be given powers to temporarily detain migrants, but not the authority to arrest them. They would only be permitted to detain migrants for “minutes, not even hours” while they hand them over to border agents.

He said troops could help protect the border agents with shields and batons, but would be unarmed.

Two U.S. officials, speaking on the condition of anonymity, said a memo signed by White House Chief of Staff John Kelly gave Mattis the authority to protect immigration agents and if necessary, could include actions like use of lethal force, crowd control, temporary detention and cursory searches.

Border patrol agents are subject to Federal law and courts for their actions (such as the BP agent who killed a woman recently).  Soldiers are subject to the military courts, and as my Criminal Law professor, a former JAG, put it:  "Military justice is to justice what military music is to music."

Military personnel should not be deciding what "reasonable force" is in law enforcement situations, because they aren't trained in it.  It's as unfair to the soldiers as it is to the immigrants, legal or illegal.  "Protect" Border Patrol agents?  That concept has a different meaning under Federal criminal law than it does in a military court-martial, a meaning far more complex than what would apply under the UCMJ.  Which applies to the soldier?  It won't be the one most convenient to the soldier, but the one most convenient to the government.  Regard the murders of Jamal Khashoggi, who have been all but promised a fair trial before their executions.  Very convenient for the Royal Family, but not exactly just.  Will soldiers be excused for beating women and children because they crossed our border?  Or will they be prosecuted as pawns in a game played by others?

Either way, this is very bad.  And the situation is only getting worse as Trump and his legal advisers decided he is a king, thanks in large part to the Supreme Court's acquiescence on Trump's final version of a Muslim ban.  The Court made the mistake of thinking they were dealing with a rational person, the same mistake the media is slowly realizing they have made.  The media is waking up to the fact that even reporting Trump's statements is being a megaphone to his lies.  Will the Court decide it is being used to justify Trump's tyranny?  Because that is what we are seeing in action, more as he feels cornered by the House Democrats and whatever Mueller has in store, not to mention (the prosecutors haven't forgotten) those tax fraud allegations.

Adding:

"He said troops could help protect the border agents with shields and batons, but would be unarmed."

That's not the way Military Times is reporting it:

The White House late Tuesday signed a memo allowing troops stationed at the border to engage in some law enforcement roles and use lethal force, if necessary — a move that legal experts have cautioned may run afoul of the Posse Comitatus Act.

The new “Cabinet order” was signed by White House Chief of Staff John Kelly, not President Donald Trump. It allows “Department of Defense military personnel” to “perform those military protective activities that the Secretary of Defense determines are reasonably necessary” to protect border agents, including “a show or use of force (including lethal force, where necessary), crowd control, temporary detention. and cursory search.”

However an earlier “decision memo” that came to the same recommendations that were contained in the “cabinet memo” was signed by President Trump, according to documents obtained by Newsweek.

The memo signed by Kelly says:

Credible evidence and intelligence indicates that migrant caravans originating from Central America and moving toward the southern border of the United States...may prompt incidents of violence and disorder that could threaten U.S. Customs and Border Protection and other United States personnel and prevent them from performing the federal functions necessary to secure and protect the integrity of the southern border.

This sounds about as fact-based as the Gulf of Tonkin resolution, and about as legally sound as the trespassing case brought against the Bundys in Oregon ("preventing them from performing the federal functions" is not exactly grounds for summary execution).  This memo explicitly authorizes lethal force, or "going Israeli" on unarmed people who might throw stones.  Kelly is not in the military chain of command, so it's dubious he has the authority asserted by this memo over the military, or that anyone, including the President, has the authority to suspend posse comitatus in these circumstances.  That the White House is taking the Supreme Court decision on the Muslim ban and running with it for all they can get, is obvious.  Whether they will be allowed to get away with it, is an open question.  As Military Times put it:

Some of those activities, including crowd control and detention, may run into potential conflict with the 1878 Posse Comitatus Act. If crossed, the erosion of the act’s limitations could represent a fundamental shift in the way the U.S. military is used, legal experts said.

That is a fundamental shift this country doesn't need.  We shall see what the courts say.  We already know what Trump says:

There are a lot of CRIMINALS in the Caravan. We will stop them. Catch and Detain! Judicial Activism, by people who know nothing about security and the safety of our citizens, is putting our country in great danger. Not good!

— Donald J. Trump (@realDonaldTrump) November 21, 2018

Next stop:  divine right of kings.