Irredentism – Wikipedia

Irredentism is a political and popular movement whose members claim (usually on behalf of their nation) and seek to occupy territory which they consider “lost” (or “unredeemed”), based on history or legend. The scope of this definition is occasionally subject to terminological disputes about underlying claims of expansionism, owing to lack of clarity on the historical bounds of putative …

https://en.wikipedia.org/wiki/Irredentism