My Empire is rivaling a neighbouring weaker Empire, let's call it Empire A, who has a defensive pact with Empire B, a much larger Empire. I recently had a war against Empire B, which ended in a truce. I plan to vassalise Empire A in order to secure my borders from invasion. However, this may cause a war against both Empire A and Empire B, a conflict I wish to avoid, due to their defensive pact. Will my war against Empire A lead to war against Empire B due to their defensive pact, despite the truce between my Empire and Empire B?
I know that truces take place after a war between two belligerents and prevent conflict between them for 10 years, but I wonder if it's possible for a Human player and the AI to break them. Also, I want to the repercussions of breaking a truce.