Military tribunals (also called military commissions) are courts, established by law, whose jurisdiction extends to certain defined classes of suspects. They are composed of military officers (at least five, the minimum number of members of a general court martial) and have certain similarities to other military judicial procedures. However, commission proceedings may be closed to the public; the accused has no right to trial by jury; rules of evidence are less stringent than in federal cases; findings of guilt and imposition of death penalty do not require unanimous approval; there is no right to appeal; and death penalties may be immediately imposed. They may be convened within the confines of the United States and its dependencies, aboard naval vessels, and in such other places as the United States may exercise effective political control (e.g., occupied enemy territory).
Authority for the establishment of such commissions ultimately lies in the United States Constitution. Article I, section 8, provides (in part) that Congress shall have the power to "define and punish piracies and felonies committed on the high seas, and offenses against the law of nations." This authority is separate from those articles governing the establishment of federal courts and their jurisdiction.
Generally speaking, the establishment of military tribunals is restricted to time of war or national emergency. (There are historical exceptions. Those persons--e.g., pirates, slavers, land raiders--whose acts made them enemies of civilized nations were subject to immediate trial and execution, without reference to formal courts or appellate procedures. Furthermore, in the United States during the nineteenth century, military commissions tried certain offenses incident to native American uprisings and depredations against civilian persons and property.)
Their American history extends back to the eighteenth century. For example, during the war of the American revolution, Major John Andre, a British officer arrested on suspicion of espionage, was tried by military commission, convicted and executed. U.S. Supreme Court cases make reference to military tribunals in the war of 1812, most notably in their allegedly high-handed treatment of the accused. During the Mexican War (1846-1848), "commissions tried guerrilla fighters and other resisters who were not part of the Mexican Army." During the Civil War, President Lincoln authorized trial by commission for all rebels in Federal territory. An estimated 4,000 Confederate sympathizers and enemy operatives were tried under these rules. Military commissions enforced the writ of the United States in Philippines during the Moro insurrection. And, of course, they were the judicial instruments for the punishment of alleged Japanese war criminals after the Pacific war ended in 1945.