The United States Constitution:
Due process is guaranteed by the 5th Amendment (1791).
"[C]ruel and unusual punishment" is outlawed in the 8th Amendment (1791).
Torture is prohibited by federal law in Title 18 of the United States Code, Part I, Chapter 113C, § 2340A. Torture:
(a) Offense.— Whoever outside the United States commits or attempts to commit torture shall be fined under this title or imprisoned not more than 20 years, or both, and if death results to any person from conduct prohibited by this subsection, shall be punished by death or imprisoned for any term of years or for life. (b) Jurisdiction.— There is jurisdiction over the activity prohibited in subsection (a) if— (2) the alleged offender is present in the United States, irrespective of the nationality of the victim or alleged offender.Torture is defined in Title 18, Part I, Chapter 113C, § 2340. Definitions:
“torture” means an act committed by a person acting under the color of law specifically intended to inflict severe physical or mental pain or suffering (other than pain or suffering incidental to lawful sanctions) upon another person within his custody or physical control....A war crime is defined in Title 18, Part I, Chapter 118, § 2441. War crimes (c):
(c) Definition.— As used in this section the term “war crime” means any conduct—Among other instruments and treaties of international law, the United States is party to:(1) defined as a grave breach in any of the international conventions signed at Geneva 12 August 1949, or any protocol to such convention to which the United States is a party;(2) prohibited by Article 23, 25, 27, or 28 of the Annex to the Hague Convention IV, Respecting the Laws and Customs of War on Land, signed 18 October 1907;(3) which constitutes a grave breach of common Article 3 (as defined in subsection (d)) when committed in the context of and in association with an armed conflict not of an international character; or(4) of a person who, in relation to an armed conflict and contrary to the provisions of the Protocol on Prohibitions or Restrictions on the Use of Mines, Booby-Traps and Other Devices as amended at Geneva on 3 May 1996 (Protocol II as amended on 3 May 1996), when the United States is a party to such Protocol, willfully kills or causes serious injury to civilians.
(d) Common Article 3 Violations.—
(1) Prohibited conduct.— In subsection (c)(3), the term “grave breach of common Article 3” means any conduct (such conduct constituting a grave breach of common Article 3 of the international conventions done at Geneva August 12, 1949), as follows:(A) Torture.— The act of a person who commits, or conspires or attempts to commit, an act specifically intended to inflict severe physical or mental pain or suffering (other than pain or suffering incidental to lawful sanctions) upon another person within his custody or physical control for the purpose of obtaining information or a confession, punishment, intimidation, coercion, or any reason based on discrimination of any kind.(B) Cruel or inhuman treatment.— The act of a person who commits, or conspires or attempts to commit, an act intended to inflict severe or serious physical or mental pain or suffering (other than pain or suffering incidental to lawful sanctions), including serious physical abuse, upon another within his custody or control...
(D) Murder.— The act of a person who intentionally kills, or conspires or attempts to kill, or kills whether intentionally or unintentionally in the course of committing any other offense under this subsection, one or more persons taking no active part in the hostilities, including those placed out of combat by sickness, wounds, detention, or any other cause...
(G) Rape.— The act of a person who forcibly or with coercion or threat of force wrongfully invades, or conspires or attempts to invade, the body of a person by penetrating, however slightly, the anal or genital opening of the victim with any part of the body of the accused, or with any foreign object.
(H) Sexual assault or abuse.— The act of a person who forcibly or with coercion or threat of force engages, or conspires or attempts to engage, in sexual contact with one or more persons, or causes, or conspires or attempts to cause, one or more persons to engage in sexual contact....
The Universal Declaration of Human Rights,One of the major pieces of international law, the The UN Convention Against Torture (1984/entered into force 1987) was signed by Ronald Reagan in April 1988 and ratified by the United States in October 1994. Ratification indicates a legally binding commitment by signatories to follow and uphold the content and spirit of the international law. Drawing out some relevant articles, the Convention states:
The Geneva Conventions,
The American Convention on Human Rights (signatory only),
The International Covenant on Civil and Political Rights, and
The UN Convention Against Torture.
Article 1.1:More international law on human rights can be found here, much of which has been promoted by the United States and to which the U.S. is party.
For the purposes of this Convention, the term 'torture' means any act by which severe pain or suffering, whether physical or mental, is intentionally inflicted on a person for such purposes as obtaining from him or a third person information or a confession, punishing him for an act he or a third person has committed or is suspected of having committed, or intimidating or coercing him or a third person, or for any reason based on discrimination of any kind, when such pain or suffering is inflicted by or at the instigation of or with the consent or acquiescence of a public official or other person acting in an official capacity. It does not include pain or suffering arising only from, inherent in or incidental to lawful sanctions...
Articles 2.1, 2.2, 2.3:
1. Each State Party shall take effective legislative, administrative, judicial or other measures to prevent acts of torture in any territory under its jurisdiction.
2. No exceptional circumstances whatsoever, whether a state of war or a threat of war, internal political in stability or any other public emergency, may be invoked as a justification of torture.
3. An order from a superior officer or a public authority may not be invoked as a justification of torture...
Articles 4.1 and 4.2:
1. Each State Party shall ensure that all acts of torture are offences under its criminal law. The same shall apply to an attempt to commit torture and to an act by any person which constitutes complicity or participation in torture.
2. Each State Party shall make these offences punishable by appropriate penalties which take into account their grave nature...
1. Each State Party shall take such measures as may be necessary to establish its jurisdiction over the offences referred to in article 4 in the following cases:
(a) When the offences are committed in any territory under its jurisdiction or on board a ship or aircraft registered in that State;
(b) When the alleged offender is a national of that State;
(c) When the victim is a national of that State if that State considers it appropriate...
Each State Party shall ensure that its competent authorities proceed to a prompt and impartial investigation, wherever there is reasonable ground to believe that an act of torture has been committed in any territory under its jurisdiction.
The United States Declaration of Independence (1776) states that,
"We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness..." [my italics].This sentence, written by Thomas Jefferson, is a direct response to the absolutist notion of a Divine Right of Kings, of an executive power beholden to no authority, no law, and no people. The Declaration of Independence and the French Declaration of the Rights of Man and Citizen are considered the two founding political documents of human rights.