Yeah but what qualifies as a threat to the war? Does it explicitly have to be something that risks revealing the existence of magic (like the aforementioned Cthulhu) or will massive collateral damage merely be enough? 

Cuz uh, there's no way nobody saw Saber and Rider fighting. Or Iskander and Gilgamesh. Or Gilgamesh and Lancelot. Really, any fight that didn't take place in the forest, the docks or the school/park should've been conspicuous as fuck even at night. And there's no way people are going to just ignore our batch of clearly magical supermen duking it out at the river, so a threshhold should be established.