Unlike civil law which is a combination of codification and legal precedent (prior decisions), the Uniform Code of Military Justice (UCMJ) established and revised by congressional actions (the US Constitution gives some powers to Congress to establish laws and regulate our military operations, and other "executive" powers to the president to utilize the military in accordance with national security needs.), is a codification of law which establishes the rules for the independent operation, control, and command of the US military’s four branches.
There are rules for military conduct for each of the military departments, covering each of the military branches. The Coast Guard, which is now a military branch in Homeland Security, has a Personnel Manual with conduct rules, but is also bound by the UCMJ.
Though it is a U.S. law, because of the various Status of Forces Agreements (SOFAs) negotiated, in conjunction with the State Department, with each country in which we have a military presence, it is one of the few that explicitly can apply beyond U.S. borders (depending on when and where the U.S. military is operating).
(Never heard of such a thing as a Status of Forces Agreement (SOFA? ) The Department of Defense policy is “to protect, to the maximum extent possible, the rights of U.S. personnel who may be subject to criminal trial by foreign courts and imprisonment in foreign prisons. Therefore, before the U.S can establish a physical presence in a foreign country - and, though the DoD has lost count, at latest estimate, there are over 800 of them, the State Dept. and the DoD “gang up” on the target country’s leaders who LOVE to experience American dollars flowing from bases in their country –(and maybe even a below-the-table pay-off) work up agreements that generally favor UCMJ justice for Americans over “local” justice.
With the rape and murder of an Okinawain woman by a suspected civilian contractor, who also was an ex-Marine, in 2016 the U.S./Japanese SOFA had to be re-negotiated for the first time since its inception in 1960. That has spurred a small upheaval in both the DoD and the State Dept.)
But, back to the popular perception that, because of the words “Court Marshal,” the UCMJ is to be dreaded.
What is a Court Marshal but a trail for a serious violation of the UCMJ, up to and including robbery, rape and murder.
Instead of a judge and 12-member jury, there is an appointed senior-member panel of officers (to include a senior enlisted member if so requested by the accused) of three to five military members that perform the functions of judge and jury.
The accused MUST be represented by legal council (which is typically an appointed legal representative from the Judge Advocate General (JAG) office…sometimes somewhat equivalent to a Court-appointed lawyer, or he/she can even pay for a UCMJ-knowledgeable/experienced civilian lawyer.
Unless there are “extenuating circumstances,” i.e, classified or maybe politically explosive information to be presented, civilians can generally witness the proceedings.
Can a person be convicted of the same crime in both civilian and military court? Basically, yes, just as state and federal courts can try a person for different violations of the same incident. The reason is that military and civilian courts are fundamentally separate systems with their own sets of requirements. As a result, the constitutional right to no double jeopardy or no double punishment is preserved by having separate trials in the two different systems. Both the federal government (which usually represents military interests) and a state (or government) may prosecute someone for the same conduct.
Most Constitutional protections have equivalency under the UCMJ, except free speech and military-critical topics such as acting disrespectfully to a superior officer, insubordinate conduct, willful disobedience, conduct unbecoming an officer, and conduct prejudicial to good order (bringing disrepute onto themselves and the service) and more. But these later rules are more accurately described as codes of conduct than limits to free speech.
Less-than court marshal, Article15 (Army, Marines), “Mast” (Navy, Coast Guard) … Might be roughly equivalent to a misdemeanor prosecution in the civilian world, but under the requirements of the UCMJ.
The judge and jury is your immediate Commanding Officer, but CAN result in as much as a hard-earned rank reduction. CAN be refused in favor of a higher-stakes court marshal.
You can read the UCMJ here.