Arabia was a region that hosted a number of different cultures, some urban and others nomadic Bedouin.[12] Arabian society was divided along tribal and clan lines with the most important divisions being between the "southern" and "northern" tribal associations.[13] Both the Roman and Persian empires competed for influence in Arabia by sponsoring clients, and in turn Arabian tribes sought the patronage of the two rival empires to bolster their own ambitions.[13] The Lakhmid kingdom which covered parts of what is now southern Iraq and northern Saudi Arabia was a client of Persia, and in 602 the Persians deposed the Lakhmids to take over the defense of the southern frontier themselves.[14] This left the Persians exposed and over-extended, helping to set the stage for the collapse of Persia later that century.[15] Southern Arabia, especially what is now Yemen, had for thousands of years been a wealthy region that had been a center of the spice trade.[15] Yemen had been at the center of an international trading network linking Eurasia to Africa and Yemen had been visited by merchants from East Africa, Europe, the Middle East, India and even from as far away as China.[15] In turn, the Yemeni were great sailors, travelling up the Red Sea to Egypt and across the Indian Ocean to India and down the east African coast.[15] Inland, the valleys of Yemen had been cultivated by a system of irrigation that had been set back when the Marib Dam was destroyed by an earthquake in about 450 AD.[15] Frankincense and myrrh had been greatly valued in the Mediterranean region, being used in religious ceremonies.