02/24/12 - The Imperials only see you as a regular citizen. Attacking them is thus counted as assault on a guard. Hopefully Bethesda will fix this. The only problem would be that you might get attacked out of nowhere as well as the fact that they would be considered enemies, preventing fast travel in certain locations. Just stick to freeing their prisoners when you see them. That's enough havoc for me and I think freeing a captured Stormcloak soldier counts as 'loosing a dog of war', if you think about it.