Irredentism

Essentially, a rather egoistic and vindictive form of nationalism where one nation goes above and beyond to pursue an occupied land/territory as rightfully belonging to them. Usually, such nation will have some elaborate backstory as to why this land belongs to them and them only. Sometimes, this backstory turns out to be true, while other (if not most) times it turns out to be founded on complete lies and grandiose nonsense. Regardless of the veracity of these claims, Irredentism is known for being a very destructive ideology/policy that has been the fuel of some of the world's most brutal invasions and massacres (e.g. Israeli–Palestinian conflict, American-Aboriginal Wars, Sabra and Shatila massacre, Russo-Ukrainian War).

More often than not, Irredentism is used as an illegitimate excuse by certain political figures to justify their claims and seizing of a territory.