There was once a Holy Grail War waged by Mages and Heroic Spirits in a town of Fuyuki. However, one Mage took advantage of the chaos of World War II to steal a Holy Grail. Several decades have passed, and the Yggdmillennia family, who took upon the Holy Grail as its symbol, defected from the Mages' Association. Furious, the Association sent forces to deal with the Yggdmillennia, but were defeated. With the Holy Grail system changed, war at an unprecedented scale breaks out.
Summary taken from: MyAnimeList