In mid March, 2025, the Federal Government started sending scores of Venezuelan immigrants detained in the United States to a foreign prison in El Salvador. The move was apparently made without any due process of law, under the auspices of the Alien Enemies Act, of 1798.
The Alien Enemies Act by its own language:
Is only applicable in the case of a declared war, invasion or predatory incursion, by a nation or government.
If so, President can, by proclamation declare that all individuals associated with that nation and not naturalized citizens of the US, will be apprehended and removed from the country. Anyone so apprehended is to be brought before a judge for a full examination and hearing, and on sufficient cause appearing, an order will issue for his removal.
The Supreme Court has declared that a President's invocation of the Alien Enemies Act is not judicially reviewable. Meaning that if the President says someone has declared war, that is not arguable.
The proclamation applies by Section 1 only to members of TdA, requires by Section 5 the use of lawful means, and requires by Section 6 removal to a location consistent with applicable law.
Actions by Trump Administration:
DHS moves Venezuelan migrants in custody nationwide to detention facility in Texas prior to March 14. They are in custody in the US because no final decision has been made as to their outcome.
On March 14, Trump signs proclamation invoking Alien Enemies Act claiming invasion by Tren De Aragua, a supposed criminal organization under the supposed direction of the Venezuelan president. Proclamation is not posted till afternoon of March 15.
Evening of March 14, transferred detainees told they would be deported the next day to an unknown location. Morning of March 15, original plaintiffs file case in District Court claiming violation of several laws and the Constitution. Court notifies government and issues temporary restraining order preventing removal of plaintiffs from the country. Subsequently, the government made an EMERGENCY motion to remove the restraining order, and when the District Court denied that motion, the Supreme Court accepted an appeal against the District Courts Order and reversed that order in spite of the fact that no final decision had been yet made, and there was a pending hearing the next day.
First question is why?
There never was any emergency requiring the governments hasty actions. The detainees were detainees. They were in custody from the start. The desired haste merely meant they would be deprived of their constitutional rights.
We know that by now the issue has been more or less resolved. The plaintiffs have accepted the governments insistence on the action being brought in Texas, and the government has accepted that the plaintiffs are entitled to a hearing before being deported. So what is the big deal?
The Supreme Court took the issue out of the hands of the legitimate courts, and squashed the restraining order, which was busily being defied by the government at the very moment that the Supreme Court was squashing it.
I really recommend that you read this case for yourself.
  The opinion itself is only 3 pages. That is because it completely ignores any required statement of facts. It has been issued per Curiam which means its a joint effort. This is usually reserved for cases which are deemed straightforward, non-controversial, and unlikely to have any future impact. In this case the court invented another reason, being that nobody wants to have their name attached to it.
For some reason Justice Kavanaugh authored a concurring opinion, which is only 1 page and can be ignored.
The real meat to the case is the dissenting opinions authored by Justice Sotomayor, and Justice Jackson.
Justice Sotomayor's dissent is 17 pages, but worth every bit. She writes clearly, intelligently, and concisely. Its 17 pages because she gives the entire story and a complete legal analysis of what was done.
Justice Jackson's dissent is only 1 page, but unlike Kavanaugh's, it is a page worth reading.
So hopefully now you understand what the big deal is. The Federal Government was in the middle a directly and intentionally violating a court order, and the Supreme Court let them off the hook.