Some time in the 5th century, the Ka'bah was a place of worship for the deities of Arabia's pagan tribes. Mecca's most important pagan deity was Hubal, which had been placed there by the ruling Quraish tribe. and remained until the Conquest of Mecca by Muhammad. In the 5th century, the Quraish took control of Mecca, and became skilled merchants and traders. In the 6th century, they joined the lucrative spice trade, since battles elsewhere were diverting trade routes from dangerous sea routes to more secure overland routes. The Byzantine Empire had previously controlled the Red Sea, but piracy had been increasing. Another previous route that ran through the Persian Gulf via the Tigris and Euphrates rivers was also being threatened by exploitations from the Sassanid Empire, and was being disrupted by the Lakhmids, the Ghassanids, and the Roman-Persian Wars. Mecca's prominence as a trading center also surpassed the cities of Petra and Palmyra. The Sassanids however did not always pose a threat to Mecca, as in 575 CE they protected it from a Yemeni invasion, led by its Christian leader Abraha. The tribes of southern Arabia asked the Persian king Khosrau I for aid, in response to which he came south to Arabia with foot-soldiers and a fleet of ships near Mecca.