“Double Jeopardy” refers to the prohibition in the Fifth Amendment to the U.S. Constitution from prosecuting someone more than once for the same crime. Under the Fourteenth Amendment, double jeopardy also applies in state court. Officially, the prohibition on double jeopardy also applies if you are tried in the military, the reality is different.
The prohibition on placing an accused in double jeopardy is actually very old, dating back to ancient Athens and to the Roman Empire. The principle behind is that if someone is acquitted, we do not want the government to be able to try them again and again until the prosecutors get the result they want. When the American Founders wrote the Constitution, they added the double jeopardy concept in the Fifth Amendment (part of the Bill of Rights) and then various courts have interpreted it since. Some limitations on double jeopardy apply in all courts and some are unique to military courts. In all courts, if your conviction is overturned on appeal, you can be tried again because legally it is as if you were never tried at all (there are some exceptions to this but it is the general principle). If the government dismisses the case on the eve of trial, they can generally try you again. In the military they can go further into the trial before dismissing it and still not have double jeopardy apply (see United States v. Easton and Article 44(c) of the UCMJ). Another major difference is that in the military they can have already punished you with an Article 15 (or Captain’s Mast) and then court-martial you for the same thing! (See United States v. Gammons). This is clearly unconstitutional, but as we have seen many times, the military doesn’t care.
However, the biggest single difference is that the military will try you for alleged crimes that you were acquitted on in state court! The basis for this is something called “separate sovereigns”, the idea that the federal government and state governments are different and so you can be acquitted in one court and then tried in the other. (See U.S. v. Lanza.) The most famous example of this was the police officers in the Rodney King beating who were acquitted in a California court and then prosecuted in federal court for depriving Rodney King of his civil rights. However, this is very rare for the obvious reason that it doesn’t look fair; it looks like it violates at least the spirit of “double jeopardy.” So, federal prosecutors have an guideline that they have to follow called the “Petite Policy.” This policy states that federal prosecutors will only try someone in federal court after an unsuccessful state prosecution if three factors are met: “first, the matter must involve a substantial federal interest; second, the prior prosecution must have left that interest demonstrably unvindicated; and third, applying the same test that is applicable to all federal prosecutions, the government must believe that the defendant’s conduct constitutes a federal offense, and that the admissible evidence probably will be sufficient to obtain and sustain a conviction by an unbiased trier of fact.” [https://www.justice.gov/usam/usam-9-2000-authority-us-attorney-criminal-division-mattersprior-approvals#9-2.031] As well, the prosecution must be approved by the appropriate Assistant Attorney General of the United States! As you might guess, such prosecutions are extremely rare.
So, does the “Petite Policy” apply to the military? Nope, they apparently don’t even know it exists. Military prosecutors routinely charge service members who have been acquitted in state court and try them again in military court. They do this if it relates to an allegation of sexual assault. If it’s a different type of crime they will generally remember the concept of double jeopardy. If you are acquitted in state court be aware that the military will likely still try you if there is an allegation of sexual assault involved in any way. To deal with this situation, you need an experienced defense attorney to aggressively fight for you in military court.