The work was first performed in a five-act version on 27 November 1745 at the Grande Écurie, Versailles to celebrate the French victory at the Battle of Fontenoy.
Lydie and the shepherds persuade him that a true king should spread happiness among his people rather than wage war.
The Emperor Trajan's wife Plautine has followed him to Armenia where he is fighting to crush a rebellion by five kings.
Plautine persuades the priests of Mars and the priestesses of Venus to pray for Trajan's success in battle.
Trajan says he is not worthy of such a great honour and asks for the gods to transform it into the Temple of Happiness for all the world instead.