If I can summarize, on multiple occasions during the Federation's investigation into the Shedai, the Federation accidentally unleashed the mental energies of the Cthulhoid Old Ones--real Old Ones, remembered in the Tholian collective consciousness, they who had uplifted the Tholians into consciousness so as to enslave them--onto the contemporary Tholians, on the final time accidentally inflicting a mental violation equivalent to agonizing rape on the entire species. (This while using the enslaved Shedai to annihilate a planet.)
The Tholians sent only a fifth of their fleet to destroy Vanguard and the Shedai contained within? Such moderation.