Some argue that martial law has been declared at times during United States history, most famously during the Civil War after Congress and President Lincoln suspended habeas corpus, a legal procedure used to challenge detention. However, the Supreme Court rejected an argument that martial law was in place during the Civil War (Ex Parte Milligan, 1 U.S. 2) (4. Wall) (1866) (holding a military commission lacked authority to try an alleged Confederate sympathizer due to the absence of martial law, which only could occur where war actually forces closure of civilian courts). Debate continues over which branch of government has authority to declare martial law: i.e., whether the President as Commander-in-Chief of the military (U.S. Const. Art. II § 2) or Congress, which is the sole body of government with the power to suspend the writ of habeas corpus as granted by the Constitution (Art. I § 9), may declare martial law.

Absent a declaration of martial law, United States civilians cannot be prosecuted under a system of military law (Reid v. Covert, 354 U.S. 1) (1957) (holding unconstitutional the trial by military court of a civilian woman for murdering her husband in the military).

What is the system of military law?

United States military law is found in the Uniform Code of Military Justice (UCMJ), Title 10 of the United States Code. It establishes military legal rules and procedures applicable to individuals in the military. The military law embodied in the UCMJ applies to the armed services at home and abroad, active and non-active. The coverage of the law is extensive, ranging from insubordination to theft. Additionally, the UCMJ implements many international laws of war which apply during an armed conflict.

The UCMJ establishes a separate military court system, the "courts-martial," in which trials involving military service personnel take place. Executive orders published in the Manual for Courts-Martial implement the provisions of the UCMJ on courts-martial. Various forms of military tribunals and commissions comprise the courts-martial system. The three most common forms of military tribunals are found in the UCMJ: General Courts-Martial, Special Courts-Martial, and Summary Courts-Martial.

The President, as Commander-in-Chief, has authority to create military commissions and tribunals. The legal procedures employed by these entities must nevertheless comport with applicable law, such as the Constitution and the UCMJ. Such military commissions and tribunals can conduct proceedings against individuals, including non-United States citizens, charged with violating the laws of war.