Sacred Haven of Debauchery

I also enjoy using similar tactics. If opponents have formed defensive pacts, but not all of the pacts overlap (for example, empires X and Y dislike each other, but both are in a defensive pact with empire Z), then I attack empire X/Y (even if the real target is Z) and invite the remaining one to join the war on my side.

Here, we have such a large fleet that we don't need to check who is defending whom. Unless, of course, we're talking about an awakened empire. The war turned out to be in two stages, and after the first stage, the empire we attacked concluded a mutual defense pact with another one, but when we attacked the second time, they immediately broke the pact.