It is a manual battle. It's easiest to achieve by finding an allied army in enemy territory and ending your turn next to that army or a target you think that army will attack (riskier ending it away form the allied army, as you might get attacked instead without reinforcement). Another option is to sit next to an enemy army that is besieging an allied town (be adjacent to the town too). That way you become reinforcements to the ally on both a siege assault and a sally. I'm not sure if you get a reputation improvement if you start the battle to relieve a siege of an allied town, but for the purposes of that edict (which I wrote) I would say that would also qualify, even if it didn't result in a reputation improvement.
Ideally, I'd like to see this become a common thing for the Reich. Sending armies out to help our allies in their wars is a good way to have interesting battles, get generals traits, and induce roleplaying without further expanding the Empire.
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