In the non-miltary context, your answer depends on whether you were a party to the conversation or not, and where you and the other individuals being taped were located. Indiana is among the majority of states that apply the "one party consent rule" for tape recording conversations. This means that if you are a party to the conversation, you are permitted to tape record it without providing notification to the others--provided that every single person being taped is in a "one party state". Therefore, if you were a party to a conversation in Indiana, and all of the others being taped were also in Indiana, it is legal to tape record the conversation.
Tape recording gets tricky when you have parties in other states. There are certain states (including Illinois and Michigan) that require all parties to the conversation to consent to the recording. Those states are typically referrred to having "two party state statutes", although in actuality, every party must consent. If you tape recorded a conversation involving parties of those states, without their knowledge or permission, you can be held liable for civil and criminal penalties. In some states, it is a felony to tape record without the permission of all parties to the conversation.
Of course, law enforcement agencies can, in appropriate circumstances, get a warrant to tape-record conversations, even if they are not a party to the conversation and even if the parties are located in "two party states".
Please note that there may be additional rules and restrictions that apply in the military context (which I am not familiar with), that further restrict taping conversations. You will need to consult an attorney who is a specialist with the military for further information.
If the tape recording was legal, whether it becomes admissible evidence is subject to the evidence rules, any special rules that apply to the military, as well as other factors, such as whether other laws may have been violated as part of the taping. (For example, a number of states prohibit the use of hidden cameras.)