during the prep to declare war, hover over the X or checkmark next to the selection box for bringing them in (beneath the war declaration reason area) to see why they will or wont join OFFENSIVE wars
depending on DLC, you need 10 favors or you need to promise them land
in DEFENSIVE wars, they usually join (large modifier to do so) but sometimes don't if they are having trouble or don't trust you anymore
edit: for wars
play on a slower speed, shift-consolidate regiments (don't consolidate completely) right before a battle because 1000 stacks fight better than 888 stacks. (regular consolidation deletes regiments) (non consolidated regiments reinforce faster but shift-consolidated regiments are stronger so do it a few days before battle)
try to avoid penalties by having generals with maneuver or by letting enemies attack YOU across rivers/into mountains etc.
Also keep stacks small enough to avoid attrition but close together so you can throw everything into a fight when it happens
depending on DLC, you need 10 favors or you need to promise them land
in DEFENSIVE wars, they usually join (large modifier to do so) but sometimes don't if they are having trouble or don't trust you anymore
edit: for wars
play on a slower speed, shift-consolidate regiments (don't consolidate completely) right before a battle because 1000 stacks fight better than 888 stacks. (regular consolidation deletes regiments) (non consolidated regiments reinforce faster but shift-consolidated regiments are stronger so do it a few days before battle)
try to avoid penalties by having generals with maneuver or by letting enemies attack YOU across rivers/into mountains etc.
Also keep stacks small enough to avoid attrition but close together so you can throw everything into a fight when it happens