You cannot simply declare war without a 'Casus Belli' (CB). Common CBs include 'Claim County' (fabricated by your Chancellor), 'De Jure Claim' (for titles you are the rightful liege of), and 'Holy War' (against characters of hostile faiths). Always check the 'War Declaration' screen carefully to understand the war goal, potential allies, and expected strength of both sides. Don't bite off more than you can chew; pick winnable wars to expand your territory.
