Terminology and grammar
As a point of terminology, someone is not an "enemy of the Constitution of the United States".
No such thing exists.
Someone is an "enemy of the United States" by virtue of the definitions set forth below.
If someone has a duty of loyalty that arises from swearing an oath to uphold the Constitution of the United States, then acting as an enemy of the United States or aiding such a person, the person with that duty has breached that duty.
Also, at the time of the adoption of the U.S. Constitution, "United States" was considered a plural reference to the collective particular U.S. states that were part of the United States of America. The "United States" is now usually considered the singular proper name of the country that uses the U.S. Constitution.
Who is an enemy of the United States?
What are the criteria? Is there an explicit list? If there is a list,
how robust is it in the face of changing times?
What would a person have to do in order to be counted as an enemy of
the Constitution?
Enemies established by Congress
For the most part, "enemy" is a term of art referring to a country or organization with whom Congress has authorized the use of force (whether or not in a formal declaration of war).
For example, a country with respect to which Congress has not authorized such action, such as China, is not an enemy of the United States, even though the U.S. is often adverse to it in international affairs and is actively planning for potential war with it in the future.
As another example, anyone with the scope of the authorization for use of military force (AUMF) passed by Congress in 2001 following the 9-11 attacks is an "enemy" of the United States so long as they are within the scope of the AUMF and it is not repealed.
A terrorist organization or member of it, so declared by the State Department under authorization from Congress, or a sanctioned country determined by Congress or the State Department under authorization from Congress in a context comparable to a terrorist designation or authorization of use of force against it, might or might not be an "enemy" as context suggests. This is a more gray area.
Enemies by violent or imminent violent conduct against the U.S.
The definition of an "enemy" of the United States also includes countries or organizations actually engaged in the use or planned imminent use of force against the United States government even if Congress has not yet taken action.
For example, Japan was an enemy of the United States the moment that someone learns that the Pearl Harbor attack will go forward, even before the declaration of war that followed that attack, and the Confederacy was an enemy of the United States when shots were fired at Fort Sumpter, even before Congress commenced the U.S. Civil War.
Similarly, people plotting an imminent coup of the United States government or carrying out a coup, are "enemies of the United States" when the plot is discovered, even if Congress has not yet taken legislative action declaring them to be enemies of the United States.
Indeed, U.S. treason and sedition statutes makes it a crime not to report such a plot if it is discovered, with failure to do so potentially constituting aid to an enemy of the United States. But exactly what triggers this duty is more clear on paper than in the ambiguous world of reality where events can unfolds bit by bit.
Enemies v. criminals
The line between "enemies" of the United States and mere criminals who have targeted government institution in the context of domestic enemies is less obvious and the relevant officials charged with enforcing U.S. law and the courts have tread carefully in that area, resorting to declaring someone to be an enemy of the United States only in the clearest of circumstances, such as the January 6, 2021 storming of the U.S. Capitol with an intent to interfere with the Congressional count of electoral votes following the 2020 election (which one court has held constituted a breach of the duty to uphold the constitution disqualifying someone from office).
Generally, the distinction comes down to ambition. Someone wanting to bring about regime change or start a war with the U.S. from a foreign country or a civil war is an "enemy" of the U.S., while someone with less lofty goals like being fed up and wanting to express their unhappiness, or wanting to steal something, is not an "enemy" of the United States.
An arguably inane person or child whose desire to do things that would make them an "enemy" of the United States that are so delusional that they cannot be taken seriously, is also a mere criminal or mentally incompetent wrongdoer, and not a true enemy of the United States.
Definitions for oath takers
Before one takes an oath to defend the Constitution against enemies
foreign and domestic, does one receive instruction on how to identify
such an enemy?
If there is instruction, is it consistent across the different types
of people who take such an oath?
Every elected official, officer of the government, and judge must take an oath of loyalty to uphold the Constitution of the United States. I even had to, for example, in connection with being admitted to the practice of law. Every military officer, not matter how junior, must also take such an oath.
No such instruction is given to people who take the oath. The legal burden is on someone who takes such an oath to figure it out on their own in the spirit of "ignorance of the law is no excuse."
Enforcement and application
Is there some kind of due process whereby one gets officially
designated as such? Is there a body, an organ of the state responsible
for this process or making this designation?
If evidence is required, what is the bar that needs to be met?
Whether or not someone is an enemy of the United States or aiding an enemy of the United States is something that either is or is not true factually in the context of a case. There is no one stop shopping list that sets for who is and is not an enemy of the United States.
How it is determined depends upon how the matter presents itself in the legal system. Usually, it involves someone alleged to be or to be aiding such a person, rather than an advanced administrative determination for which everyone is given notice in advance.
The evidentiary standards and process depends upon the way that the issue is raised: a criminal prosecution for treason or sedition, a court determination if someone is eligible to hold office in light of the 14th Amendment prohibition on certain people holding public office, a writ of habeas corpus for someone detained as an enemy combatant, or some other context.
The question of who is an enemy at any given time is not exclusively a judicial question, even though it is a justiciable issue that courts may resolve in the proper cases.
Congress rather than a court could adjudicate the issue as well in its role to determine whether someone is eligible and qualified constitutionally to serve in Congress, or in the context of an impeachment proceeding.
Likewise, the executive branch frequently determines who an enemy is in the course of conducting military action and determining which potential targets of military action are lawful. This is often done without any due process.
Sources of law
In general
A lot of the relevant law comes from sources including:
(1) The U.S. Constitution (see below), and the case law interpreting it,
(2) Statutes enacted by Congress,
(3) litigation of the "enemy combatant" cases following 9-11 and during WWII,
(4) from post-U.S. Civl War 14th Amendment disqualification to serve in public office cases, including one recent decision on point,
(5) from the handful of treason and sedition cases producing legal precedents, see, e.g., Cramer v. US, 325 U.S. 1 (1945),
(6) from historical context regarding word meaning when the relevant laws and the U.S. Constitution was adopted,
(7) from espionage cases,
(8) the law governing when the war power to use military force against someone or some country or group can be recognized (this is because military actions and war powers are only authorized to be used primarily (setting aside harm to people that is collateral damage that is not a war crime) against an enemy of the United States or someone acting in concern with an enemy of the United States), and
(9) some of these definitions also track definitions codifying international law found in the Geneva Conventions (especially Geneva III) which are intentional treaties to which the U.S. is a party.
Pertinent U.S. Constitution language
Article I, Section 8 of the United States Constitution states in the pertinent part:
The Congress shall have Power To . . .
To define and punish Piracies and Felonies committed on the high Seas,
and Offences against the Law of Nations;
To declare War, grant Letters of Marque and Reprisal, and make Rules
concerning Captures on Land and Water;
To raise and support Armies, but no Appropriation of Money to that Use
shall be for a longer Term than two Years;
To provide and maintain a Navy;
To make Rules for the Government and Regulation of the land and naval
Forces;
To provide for calling forth the Militia to execute the Laws of the
Union, suppress Insurrections and repel Invasions;
To provide for organizing, arming, and disciplining, the Militia, and
for governing such Part of them as may be employed in the Service of
the United States, reserving to the States respectively, the
Appointment of the Officers, and the Authority of training the Militia
according to the discipline prescribed by Congress;
. . .—And To make all Laws which shall be necessary and proper for
carrying into Execution the foregoing Powers, and all other Powers
vested by this Constitution in the Government of the United States, or
in any Department or Officer thereof.
Article II, Section 2 of the United States Constitution states in the pertinent part:
The President shall be Commander in Chief of the Army and Navy of the
United States, and of the Militia of the several States, when called
into the actual Service of the United States[.]
Article II, Section 4 of the United States Constitution states:
The President, Vice President and all civil Officers of the United
States, shall be removed from Office on Impeachment for, and
Conviction of, Treason, Bribery, or other high Crimes and
Misdemeanors.
Section 3 of Article III of the United States Constitution states:
Treason against the United States, shall consist only in levying War
against them, or in adhering to their Enemies, giving them Aid and
Comfort. No Person shall be convicted of Treason unless on the
Testimony of two Witnesses to the same overt Act, or on Confession in
open Court.
The Congress shall have Power to declare the Punishment of Treason,
but no Attainder of Treason shall work Corruption of Blood, or
Forfeiture except during the Life of the Person attainted.
Article IV, Section 2 of the United States Constitution states in the pertinent part:
A Person charged in any State with Treason, Felony, or other Crime,
who shall flee from Justice, and be found in another State, shall on
Demand of the executive Authority of the State from which he fled, be
delivered up, to be removed to the State having Jurisdiction of the
Crime.
Article IV, Section 4 of the United States Constitution states:
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.
The third paragraph of Article VI of the United States Constitution states:
The Senators and Representatives before mentioned, and the Members of
the several State Legislatures, and all executive and judicial
Officers, both of the United States and of the several States, shall
be bound by Oath or Affirmation, to support this Constitution; but no
religious Test shall ever be required as a Qualification to any Office
or public Trust under the United States.
Section 3 of the 14th Amendment to the United States Constitution states:
No person shall be a Senator or Representative in Congress, or elector
of President and Vice-President, or hold any office, civil or
military, under the United States, or under any State, who, having
previously taken an oath, as a member of Congress, or as an officer of
the United States, or as a member of any State legislature, or as an
executive or judicial officer of any State, to support the
Constitution of the United States, shall have engaged in insurrection
or rebellion against the same, or given aid or comfort to the enemies
thereof. But Congress may by a vote of two-thirds of each House,
remove such disability.
Selected pertinent statutes and case law
Congress indeed did pass legislation to remove that disability from all but about 500 people involved in the Confederacy in the U.S. Civil War who would otherwise have been disqualified by this provision 26 years after the 14th Amendment was adopted, but not before some actions were taken pursuant to Section 3 of the 14th Amendment and related legislation based by Congress to enforce that section of the 14th Amendment shortly after the 14th Amendment was adopted.
Definitions of treason and sedition crimes under federal law (states also have such statutes) are found at:
18 U.S. Code Chapter 115 - TREASON, SEDITION, AND SUBVERSIVE ACTIVITIES
§ 2381. Treason
§ 2382. Misprision of treason
§ 2383. Rebellion or insurrection
§ 2384. Seditious conspiracy
§ 2385. Advocating overthrow of Government
§ 2386. Registration of certain organizations
§ 2387. Activities affecting armed forces generally
§ 2388. Activities affecting armed forces during war
§ 2389. Recruiting for service against United States
§ 2390. Enlistment to serve against United States
The federal treason statute states:
Whoever, owing allegiance to the United States, levies war against
them or adheres to their enemies, giving them aid and comfort within
the United States or elsewhere, is guilty of treason and shall suffer
death, or shall be imprisoned not less than five years and fined under
this title but not less than $10,000; and shall be incapable of
holding any office under the United States.
18 USC Section 2381.
Thus, to sell to, or
provide arms or munitions of war, or military stores, or supplies,
including food, clothing, etc., for the use of the enemy, is within
the penalty of the statute. And to hire, sell, or furnish boats,
railroad cars, or other means of transportation, or to advance money,
or obtain credits, for the use and support of a hostile army is
treasonable. It is equally clear that the communication of
intelligence to the enemy by letter, telegraph, or otherwise, relating
to the strength, movements, or position of the army, is an act of
treason. These acts, thus briefly noted, show unequivocally an
adherence to the enemy, and an unlawful purpose of giving him aid and
comfort.
U.S. v. Bond (2014)
Also rebellion or insurrection is a crime defined as follows:
Whoever incites, sets on foot, assists, or engages in any rebellion or
insurrection against the authority of the United States or the laws
thereof, or gives aid or comfort thereto, shall be fined under this
title or imprisoned not more than ten years, or both; and shall be
incapable of holding any office under the United States.
18 USC Section 2383
Sedition (actually seditious conspiracy) is defined as:
If two or more persons in any State or Territory, or in any place
subject to the jurisdiction of the United States, conspire to
overthrow, put down, or to destroy by force the Government of the
United States, or to levy war against them, or to oppose by force the
authority thereof, or by force to prevent, hinder, or delay the
execution of any law of the United States, or by force to seize, take,
or possess any property of the United States contrary to the authority
thereof, they shall each be fined under this title or imprisoned not
more than twenty years, or both.
18 USC Section 2384.
Two pertinent Korean war related cases are Thompson v. Whittier, 185 F. Supp. 306, 314 (D.D.C. 1960) (three-judge district court) and Martin v. Young, 134 F. Supp. 204, 207, 208 (N.D. Cal. 1955).
For example, Martin was a habeas proceeding by a serviceman who was being court-martialed for allegedly aiding the enemy while, a prisoner of war in Korea. He had reenlisted after the term of enlistment in which his crime was alleged to have been committed, and was still in the armed forces at the time of his court-martial. A jurisdictional provision of the Uniform Code of Military Justice (since repealed or amended) provided for courts-martial for certain military personnel whose terms had expired, but only if they were not triable for the same offense or a greater one in a civil court. The district court found that Martin could have been tried in a civil court on (among other charges) a charge of treason, and therefore that the court-martial lacked jurisdiction to try him oh a lesser, aiding the enemy charge.
Implicit in the court's decision was the view that treason could have been established for giving aid and comfort to the enemy during the undeclared war in Korea. See, generally, here.
There are other statutes, and there is other case law, which is also relevant to the question.
