One doesn't swear an oath to the UCMJ, those are the initials of the Uniform Code of Military Justice, its basically commonlaw codified for application in military jurisdiction (on military bases). The oath sworn in military court by witnesses before testifying is the same as what is sworn (or promised) in any court, "to tell the truth, the whole truth and nothing but the truth".
The UCMJ has relatively unimportant variations in trial procedure, for example everyone is absolutely guaranteed court-appointed counsel in military court, but it has to be a military attorney. Commonlaw doesn't have much weight, so there's less reliance on precedent and more focus on the specific interpretation of the statutory language in whatever law applies.
For a critical lefty, a military trial is very distressing, there's no emotional appeal, no consideration for a criminal defendant's socio-economic background, whether he was raised in a 'broken' family, if his mother had to work or the level of education his parents had, it makes no difference if he lived all his life in crime-infested gang neighborhoods and suffered lots of peer pressure, his ethnicity makes no difference either.
Military law is a lot like Roman law, like the French system of codified law. All the applicable laws are written down as articles or statutes and strictly applied. To defend oneself in a military proceeding requires showing some difference between what is specified in the stature and the conduct to which it is being applied. In Manning's case the whole defence must rely on this explicit reference to an "enemy" as the recipient of the classified documents, likely there will be some discussion of whether Wikileaks is a news organization or if a business can fit under the enemy definition.