My ally doesn't join war but is still my ally. Huh?

[Fisherking]

It may be more trouble than it is worth. You will be lucky if -5 is all you have to pay.

[Slicendice]

Initially you get a big hit with the faction and everyone else but it eventually reduces to -5 iirc. Or maybe the -5 is for a broken treaty ie. military access that ends when you must declare war on the faction you gave it to.