The Tholian Hive Revolt was a conflict involving the Tholians, and included some combat inside nebulae.

In 2372, Commander William T. Riker read up on every log and historical record pertaining to conflicts inside nebulae, including the Tholian Hive Revolt.

It may have been during this conflict that the Tholians attempted to collect volatile gases from a class-9 nebula and use it as a weapon. (TNG - Slings and Arrows eBook: A Sea of Troubles)