There is a confounding factor when playing as a Catholic faction in the form of the Pope, who threatens to excommunicate whoever he perceives as the aggressor.

For instance, playing the Redux Mod as the French, I had stolen Burgundy (the stupid Burgundians had left it unguarded). Anyway, some turns later the Lombards treacherously attacked and were defeated. The next turn I responded by invading Lombardy; of course the Pope gives me an ultimatum: leave Lombard territory within two years and refrain from attacking for 10. So I subsequently moved my army from Lombardy to Milan (for the pillage and to reduce their buildings), then back to Burgundy before offering a ceasefire.

You could try inviting an attack, if you have a low value province you can leave lightly garrisoned. Or use agents in numbers for their nuisance value: to incite a rebellion or assassinate any target of opportunity.

Also, have a plan of what you intend to achieve once it kicks off. Against smaller factions, going directly for the King, both strategically and on the battlemap, can reap rewards.