Nathiri
Commander
Locked alliances can lead to all sorts of trouble. It gets really fun in the Napoleonic Europe Conquest Scenario, because there's so many of them and a whole bunch of civilizations to stir up trouble. Sometimes it's not even the player nation who creates the locked alliance war, iirc. There's something really fun about creating a mutual protection pact with Portugal and then declaring war on the Dutch.
England is in a locked alliance with the Dutch, so they'll declare war on you. After they land a few troops and attack, Portugal has to declare war on England (even with their locked alliance). Naples might also declare war on you and attack if you're Austria (Naples is allied with England) and then Portugal declares on them too.
Next thing you know, The British are so busy fighting their allies they forget all about the French.
I remember playing Fall of Rome several years ago and I accidentally got Eastern Rome and Western Rome to declare war on each other using this tactic. At the time I didnt realise how exactly I did it.