Who appoints the jury in the military justice system?
In the military justice system the major player is called the convening authority. The convening authority is a unit or organization commander who decides whether to refer a case to a court martial, decides what type of court martial, and then appoints the members of the court (i.e., the jury).
The convening authority is rarely a lawyer, although they are required to obtain the advice of a lawyer (the staff judge advocate) in certain cases, such as whether to convene a general court-martial. UCMJ Art. 34, 10 U.S.C. 834.
The convening authority has many of the powers conferred upon a district attorney in the civilian world, and some powers that a district attorney does not have - such as picking the jury. A convening authority will normally pick mature and responsible officers for a court-martial, which usually means officers with long service and command experience.
All courts martial start out with officer members, and ordinarily all members will be senior to the accused. An enlisted person has the right to request enlisted members, in which case the panel must have be at least 1/3 enlisted. UCMJ Art. 25(c), 10 U.S.C. 825(c). As with the officer members, the convening authority will usually pick enlisted members with long service and leadership experience. It is not unusual to have the court composed of senior officers and senior enlisted personnel.
An enlisted person has the opportunity to examine (voir dire) the members to see if they had any prejudices, knowledge of the case, etc. They could challenge the members for cause, and have one preemptory challenge they could use to exclude a member for any reason. A general court martial must have a least 5 members, and if the number was reduced to below 5 due to challenges, the convening authority would appoint replacement members.
A military verdict usually requires a 2/3 majority. If a jury only has 5 members, 4 members have to agree to convict; so if there is a convictions, at least one enlisted member voted to convict. Both the number of enlisted members and the number of votes required to convict are rounded up to whole persons: 1/3 of 5 equals 2; and 2/3 of 5 equals 4. (Note: A unanimous verdict is required in death penalty cases, and a majority vote for life imprisonment or confinement for more than 10 years. UCMJ Art. 52, 10 U.S.C. 852.)