Have you tried using the #[debug_handler] macro on get_all_armies? Without that macro handler errors don't tell you much more than "something isn't right".

Generally this kind of error indicates some type in the handler signature doesn't implement a necessary trait. Maybe you accidentally lost an automatically-derived trait like Send + Sync? The macro is the easiest way to check.

Try using https://docs.rs/axum-macros/latest/axum_macros/attr.debug_handler.html

It should provide a better error message.

That looks like a Extension/State mismatch to me. Do your function arguments line up with any extensions or state you are bringing in?