On the happenings in the United States:
There is an common argument that due process belongs only to citizens
Citizens deserve it, non citizens don’t
And, therefore, can be dealt with extrajudicially
That is a perfectly logical, internally consistent position
Now let’s think through its implications
IF citizens have the due process, and non-citizens don’t
THEN we have two parallel systems of justice
One slow, cumbersome, subject to open discussion and to appeal (due process)
Another swift, expedient, not really subject to discussion and effectively non-appealable (extrajudicial)
And the second one already encompasses tens of millions of non citizens living in the United States, legal and illegal, residents or not.
Now the question would be:
Which system is more convenient for those in power?
Well, the answer is obvious
The second one: and by a very, very wide margin
Quite naturally, the state and the authorities will absolutely love extrajudicial processes, that allow them to deal with their political opponents and to suppress dissidence fast and with very little fuzz.
So, again, you will have two systems simultaneously.
One those in power hate (due process), and another they love (extrajudicial).
What does that mean in practice:
It means that at the every possible occasion, and under every possible pretext the space of due process will shrink, and the space of the extrajudicial process will be expanded
Bit by a bit
There will be new pretexts, new reasons, new “emergencies”, and new procedural (not necessarily even legal) loopholes, to expand the space of the extrajudicial1.
So, in a few iterations you will be having an ocean of extrajudicial process with - may be - some islands of due process scattered in between
That is totally normal and happens all the time
In fact, extrajudicial processes are typically being limited at first - to the lowly, undesirable and dissident groups - and only then, gradually, being extended up the social ladder, up to the very elites.
So, the existence of extrajudicial proceedings against those below presents a mortal danger for the due process high above - if not immediately - than within just a few iterations.
Which is the main - or perhaps the only reason - why some countries have due process in the first place.
It is not that social elites just decide to extend it on everyone out of grace
It is that those in power want to extend it on themselves, so that they could not be killed, jailed or exiled extrajudicially, just out of political expediency
And once they do, it turns out that keeping two systems of justice at once is becoming impossible
One of them will eventually devour the second2.
With citizens, you have to engage into the long, tiresome litigations, that may easily last for years, and can actually outlast your administration.
With non-citizens, you can just ship them off into a Salvadoran supermax and nobody will be able to get them out from there.
Not necessarily legal. But a great procedural invention.
Consider two neighbouring countries: France and England
Both of them used to rely on some mix of due process and extrajudicial procedures
(The latter being extremely useful to deal with political opponents)
Yet, through the 17-18th c. their trajectories diverged
In England, the due process won, and old extrajudicial procedures died for everyone
(from the commoners, and up to the social elites)
In France, the extrajudicial process won, and won for everyone
(from the commoners, and up to the social elites)
Like yes, the pockets and the islands of due process remained even in France, but they could not do much except protesting
At the point at which due process was gone for almost everyone in France, they had what's known as "the French Revolution". Turns out the autocratic approach is not stable
Sorry, but in 21st century England due process for non-citizens is pretty limited. Not so long ago, the government summarily cancelled visas of tens of thousands of students because these were deemed to cheat at their pre-visa English tests, just because they took them at centers were cases of such cheating were found. No due process, nothing. And it's pretty much how non-citizens (from outside of the EU in particular) were dealt with during many years of Tory. Denials of visas for dads of children born to UK mothers, on the basis that Skype calls suffice to maintain a family bond, etc.