Subjects
You need to manage your subjects' loyalty more carefully, as they may not fight wars with you if they are disloyal enough, and actually make a separate peace with your enemies.

We also made it possible to instruct your subjects on how they should behave in your wars, when they are actually loyal enough to help. You can change them all with one button click, and you can make individual choices, which include the following..

Normal - The subject will take a balanced approach to military tactics.
Aggressive - The subject will take an aggressive approach to military tactics, looking to start sieges and chase units in enemy territory.
Supportive - The subject will take a supportive approach to military tactics, helping our units.
Passive - The subject will take a passive approach to military tactics, doing low-risk actions like blockading as well as defending friendly territory.
Defensive - The subject will take a defensive approach to military tactics, looking to stop enemies from doing anything in allied lands only.

France has a lot of subjects…

Speaking of subjects, we also added in the feature from the EU4: Art of War DLC, where you can start a war using one of your subjects' casus belli.