I would like to say that in reality there was a pretty high chance of that. I am sure most monarchs believed they had rights to near enough every Kingdom in their vicinity due to some prehistoric claim from a family member that was around a hundred years ago. Having said that, they also had to remember that the Pope was breathing down their backs. Upsetting him could reward you with a death sentence and so acted as a measure of restraint to most monarchs.
I would expect the more ambitious Kings throughout history believed they had some legal claim on neighbouring Kingdoms. People in power do get delusions of grandeur if history is anything to go by. I don't have any refer to go by though, so can't give you any examples.