[Figleaf23]

It might be useful to think about this in scenarios that are somehwat artificial but analytiically less complex.

For example, consider two well-known low-tech militaries that actually met competitively (e.g. Rome and the Parthians). Add ceremonial magic as a newly discovered technology to one or both sides and see what happens.

This way you can see some of the mechanics in action without having to speculate on the evolution of counter-measures, logistics and organization.