A Jedi From Earth Wikia

Daenerys Targaryen's invasion of Westeros is a military effort by Daenerys Targaryen to conquer the Seven Kingdoms and seize the Iron Throne of Westeros. Previously, House Targaryen ruled the Seven Kingdoms until they were overthrown during Robert's Rebellion two decades before Daenerys's invasion. Her father, Aerys II Targaryen, was the last Targaryen king to sit on the Iron Throne.

The rebellion resulted in the crowning of Robert Baratheonand the beginning of the Baratheon dynasty. Robert's reign was characterized by relative stability and strength, surviving through the Greyjoy Rebellion. However, Robert's death resulted in a massive civil war in the realm, the War of the Five Kings, which greatly weakened the dynasty and failed to end the growing friction between the noble houses of Westeros. Following the destruction of the Great Sept of Baelor, Cersei Lannister ascended to the Iron Throne, thus beginning the Lannister dynasty, though the Lannisters had truly ruled from the shadows since Robert's death.

After forming a pact with Yara Greyjoy and Theon Greyjoy, who oppose their uncle, Euron Greyjoy, King of the Iron Islands, and securing the allegiance of House Tyrell, the rulers of the Reach, and Dorne, the combined Targaryen fleet sails to Westeros to place Daenerys on the Iron Throne and restore the Targaryen dynasty.

(source GOT wiki)