Interesting - has the usage of 'mit' with 'kämpfen' changed over time? I haven't lived in Germany for over 25 years, but when I first learned German, we were told very directly not to use 'mit' in that way with 'kämpfen' (i.e. that 'kämpfen mit' always meant together not against one another), and I don't remember hearing any exceptions to that when I lived there. I did notice, though, that certain English grammatical constructions were creeping into German, particularly in spoken German among young people. Is this one of those cases, or were my German teachers overly pedantic about something that wasn't strictly true?
Schiller is otherwise using very archaic and/or dialectal language (anbefehlen, fodern, allwo) and yet "Kampf mit" (evidently referring to a duel, not some kind of fight together against a third party) already makes an appearance.
When some language construct is being taught as "incorrect", it often means that it's actually a natural part of the language (or maybe of a dialect) that is deemed to be wrong by so-called experts. Modern linguists are more descriptive than prescriptive, but teachers tend to keep propagating such ideas.
