(With the) the action or fact of laying down one's life for one's country in battle, especially as applied to combatants in the First World War (1914–18); also in extended use.
Origin
Late 19th century.
Definition of supreme sacrifice in US English:
supreme sacrifice
noun
(With the) the action or fact of laying down one's life for one's country in battle, especially as applied to combatants in the First World War (1914–18); also in extended use.