It seems gamey to declare a war you have no intention or capability to wage simply to cause negative game effects against another realm.
Capability shouldn't really enter the equation. It's perfectly OK to send a declaration of war even if it's simply as a show of support to your allies. It can also entail commercial embargoes, ability to arrest priests of the other realm should they preach in your lands, etc.
Intention, now, I'll give you.
