Over the weekend, President Donald Trump expressed frustration with the legal process afforded those accused of entering the country illegally.

We cannot allow all of these people to invade our Country. When somebody comes in, we must immediately, with no Judges or Court Cases, bring them back from where they came. Our system is a mockery to good immigration policy and Law and Order. Most children come without parents… — Donald J. Trump (@realDonaldTrump) June 24, 2018

Despite the President's appeal to "Law and Order," it is well-established that all those within the United States are entitled to the constitutional protections of Due Process, even those who may have entered the country unlawfully.

Let's start with the Constitution's text. The Fifth Amendment provides that "No person shall . . . be deprived of life, liberty, or property, without due process of law." Of note, this protection applies to all "persons"—not merely to citizens. This point is reinforced by the Fourteenth Amendment, which affirms that the class of "persons" entitled to constitutional protection encompasses citizens and non-citizens alike. (It does this by, among other things, noting which persons constitute citizens.)

As Steve Vladeck noted on Twitter, the Supreme Court has made quite clear that this language means what it says, and applies to aliens on U.S. soil. So, for instance, in 1982 the Court held:

Whatever his status under the immigration laws, an alien is surely a "person" in any ordinary sense of that term. Aliens, even aliens whose presence in this country is unlawful, have long been recognized as "persons" guaranteed due process of law by the Fifth and Fourteenth Amendments. Shaughnessv v. Mezei, 345 U.S. 206, 212 (1953); Wong Wing v. United States, 163 U.S. 228, 238 (1896); Yick Wo v. Hopkins, 118 U.S. 356, 369 (1886).

As the Court's citations indicate, the principle the Due Process applies to non-citizens was not a late-Twentieth Century innovation. As the Court had explained in 1896:

'These provisious are universal in their application to all persons within the territorial jurisdiction, without regard to any differences of race, of color, or nationality; and the equal protection of the laws is a pledge of the protection of equal laws.' Applying this reasoning to the fifth and sixth amendments, it must be concluded that all persons within the territory of the United States are entitled to the protection guarantied by those amendments, and that even aliens shall not be held to answer for a capital or other infamous crime, unless on a presentment or indictment of a grand jury, nor be deprived of life, liberty, or property without due process of law.

Here the Court was relying on prior precedent from 1886:

The Fourteenth Amendment to the Constitution is not confined to the protection of citizens. It says: Nor shall any State deprive any person of life, liberty, or property without due process of law; nor deny to any person within its jurisdiction the equal protection of the laws. These provisions are universal in their application to all persons within the territorial jurisdiction, without regard to any differences of race, of color, or of nationality, and the equal protection of the laws is a pledge of the protection of equal laws.

Of note, the late-Nineteenth Century Supreme Court was not particularly pro-immigrant, and yet the Court had no trouble recognizing that the Constitution's Due Process protections apply to aliens, even those accused of being unlawfully present. Indeed, one of the purposes of providing Due Process protections is to help make sure that those detained, deported, or otherwise sanctioned are, in fact, unlawfully present in the country.

Whatever one thinks about what the nation's immigration policy is, or should be, the Consitution provides that all persons in the United States are entitled to Due Process, and true respect for "Law and Order" requires respect for the Constitution.