Well, I might be making some generalizations, but the so called "Fall of the Roman Empire" (Basically a generalization on the traditional Roman Empire that most people know, est. 3rd Century BCE to 3rd century CE) resulted in the abandonment, and then ignorance, of higher military systems, procedures, philosophies, etc.

For instance, one of the most significant subjects that was forgotten, decayed, or at least less emphasized was advanced military logistics. Of course all military leaders knew that an army must be fed, but did they know how to ensure that an army of 40,000 men always had one week of rations at any given time? Or to march the same sized army 19.5 miles in a single day? By the 3rd century BCE, matters like these pertaining to military logistics were common knowledge in the Hellenistic world and became fundamental to Rome's field of military logistics.

However, after "Rome's Fall", it's evident that advanced military logistics was not a commonly-known subject. In fact, with a few exceptions, logistical problems with armies continued until at least the 15th century, a notable example being how Henry V's army was starving in less than two months after being deployed.

I don't know if this fits in with your question, but I thought the "art of higher military processes" and its decay was a notable topic to bring up.

****Edit: Note italicized means generalized.