In Crusader Kings 3 (a video game set in either the 9th or 13th century) you are required to have a Casus Belli (a justification) for going to war with another Kingdom to conquer their land. Did medieval rulers really care about "justifications" for conquering land?
While there may be more to say, you might be interested in this panel that was held just a couple of weeks ago that touched on CK3 and many of the aspects of the period, including casus belli.