Short answers, pulled from the story.
Irredentism is a policy that seeks the recovery and reunion with one country of a region currently belonging to another, typically justified by shared ethnicity or historical ownership. The term was coined from the Italian phrase Italia irredenta, meaning "unredeemed Italy," which referred to a movement after 1878 claiming parts of Switzerland and the Austro-Hungarian Empire inhabited by ethnic Italians.
Frequently cited examples include Nazi Germany's annexation of the Sudetenland in 1938, Somalia's invasion of Ethiopia in 1977 to unite ethnic Somalis in the Ogaden region, Argentina's invasion of the Falkland Islands in 1982, the Greater Serbia and Greater Croatia campaigns during the breakup of Yugoslavia in the early 1990s, and Russia's annexation of Crimea in 2014.
Irredentism aims to build a unified nation-state by annexing foreign territory on ethnic or historical grounds. Revanchism is driven by revenge for a prior defeat and seeks to recover lost land, as French nationalists aimed to reclaim Alsace-Lorraine after the Franco-Prussian War. Secession involves a territory breaking away to form an independent state, whereas in irredentism the territory merges into another existing state.
Researchers cite ethnic homogeneity in the irredentist state, discrimination against ethnic kin across the border, national identities rooted in ethnicity and culture, the economic value of the target territory, and regime type as contributing factors. Scholars David S. Siroky and Christopher W. Hale identify anocratic regimes as especially prone to irredentist conflict, while democracies are generally considered less likely to engage in it.
International law is broadly hostile to irredentism. The United Nations Charter calls for respect for established territorial borders and state sovereignty. The Organization of African Unity, the Organization of American States, and the Helsinki Final Act take similar positions. Peaceful resolutions of irredentist disputes typically result in mutual recognition of existing de facto borders rather than territorial transfers.
Irredentist movements rarely achieve their goals. Somalia came close to annexing the Ogaden region in 1977 but ultimately failed after foreign intervention. Argentina was defeated in the Falklands War in 1982. Beyond failure to win territory, irredentism often worsens conditions for the ethnic minorities it claims to protect, as host states may increase discrimination against those groups in response to the perceived threat.