To use “up in arms” was probably an unfortunate (or inapt) figure of speech to use in this context. Most here naturally think of guns when the terms “arms” is mentioned, particularly when state power is being discussed.
You are correct. It is an idiomatic expression that I’ve used for around 50 or so years (picked it up from my Dad at a young age), but I should be more careful using it in threads of this nature (although I certainly wasn’t thinking along any literal sense of the phrase).