Which Fire Emblem game introduced the 'Casual Mode' (optional permadeath)?

The Fire Emblem game that introduced Casual Mode was Fire Emblem: New Mystery of the Emblem. Released for Nintendo DS in 2010, it let fallen units return for the next battle instead of permanently leaving the army.

Casual Mode was a major change to the series’ traditional design. Earlier Fire Emblem games generally treated a defeated character as permanently dead, creating lasting consequences for tactical mistakes. New Mystery of the Emblem allowed players to choose between Casual Mode and Classic Mode, while keeping difficulty settings separate from that choice.

The feature was debated during development because permanent death had long been central to Fire Emblem’s identity. Its introduction helped make the series more approachable without removing Classic Mode. Later games, including Awakening, Fates, and Three Houses, continued to offer both approaches. New Mystery of the Emblem itself was never released officially in the West.