Regular systems of military law existed in ancient Rome, with severe penalties for such offenses as desertion. In the Middle Ages procedures were less regularized, but written codes began to appear. The origin of much military law is found in the codes and statutes enacted in England in the 17th cent. These were substantially adopted in the United States.

It was widely felt after World War II that many abuses had occurred in the administration of American military justice and that excessively severe sentences had been imposed, especially on the enlisted ranks. The armed forces responded by establishing civilian review boards, which recommended reduction of the punishment inflicted on a large percentage of those convicted (some 100,000) by general court-martial during the war. In 1951, Congress extensively revised the codes of military law enacting a uniform code of military justice for all branches of the armed services. This code placed operations more in the hands of professional lawyers and ensured fairer review procedures.

An important change permitted an enlisted person tried by a general court-martial to demand that one third of the court be composed of enlisted personnel. The uniform code defines the offenses for which a person under the jurisdiction of the armed forces may be subjected to court-martial. In addition to allowing punishments by the commanding officer, including confinement not to exceed one week, the code establishes three levels of court-martial. The summary court-martial consists of a single officer, and may impose a maximum penalty of imprisonment for one month. The special court-martial consists of at least three officers and may impose a prison sentence of up to six months. The general court-martial is composed of five members and one law officer who must be a trained lawyer admitted to practice before a state's highest court. The general court-martial may impose any authorized sentence including dishonorable discharge or death.

One of the principal differences between the procedure in court-martial and in criminal cases in civil courts is the absence of a jury. Cases are decided by a vote of two thirds or three fourths of the court, depending on the severity of the offense. For the death penalty, the vote must be unanimous. The accused is permitted to have counsel, to compel the attendance of witnesses, and to enjoy the usual protections of the law of evidence.