Fighting war with other countries in My empire

I read somewhere that it was possible to fight with any of the military unit in your empire regardless of which actually declared the war. This was not possible when i tried it as it said i was not at war with the country.

So my question is: When at war, is it possible to fight the war with any of the military unit in your empire? And how is this done if it is possible?

War decs are against countries not empires. All because one country of an empire is at war does not mean all the other countries can join in. They can however provide federated air defense if they are in the same fed.