After this, Demetrius was sent to bring into subjection the Arabs known as Nabataean, and incurred great peril by getting into regions which had no water; but he was neither terrified nor greatly disturbed, and his demeanour overawed the Barbarians, so that he took much booty and seven hundred camels from them and returned. And now Seleucus, who had once been expelled from Babylonia by Antigonus, but had afterwards succeeded in recovering the realm and was now wielding the power there, went up with an army, designing to annex the tribes on the confines of India and the provinces about Mount Caucasus. Demetrius, accordingly, expecting that he would find Mesopotamia unprotected, suddenly crossed the Euphrates and invaded Babylonia before Seleucus could stop him. He expelled from one of its citadels (there were two of them) the garrison left there by Seleucus, got it into his power and established in it seven thousand of his own men. But after ordering his soldiers to take and make booty of everything which they could carry or drive from the country, he returned to the sea-coast, leaving Seleucus more confirmed than before in his possession of the realm; for by ravaging the country Demetrius was thought to admit that it no longer belonged to his father. However, while Ptolemy was besieging Halicarnassus, Demetrius came swiftly to the aid of the city and rescued it.