Electricity might work--and that would have interesting complications when it came to lightning. (I like interesting complications, which is why I love the printing press idea.)

The thing is, I want it to be a single thing--or at least a combination of a few things that are relatively easily destroyed. I want it to be able to kind of suddenly come back in the middle of the war, taking everyone, including the magic users, by surprise. That's why the "belief" aspect, which I do usually love, doesn't work in this setting.