@the spirit of mankind Clearly the eldest son of the current ruler inherits the throne, if the king dies without a hair (very rare occurrence) the parliament steps in and selects a close relative (an uncle or cousin most of the time) which does create some tension but it's still unlikely to cause a full war.
Succession crises are incredibly rare and and lead to short succession wars at worst, and I'm pretty sure that republics also deal with civil wars and those take way longer and the country takes an extremely long amount of time to stabilize ,many countries (republics) have been in a state of civil war for the last 30 or more years, because succession is much less clear in a democratic state and it's nearly impossible to elect a new president effectively in a state of chaos.