He continued southward to the Persian Gulf, when, after his fleet escaped a tidal bore on the Tigris,[55] he received the submission of Athambelus, the ruler of Charax. He declared Babylon a new province of the Empire and had his statue erected on the shore of the Persian Gulf,[56] after which he sent the Senate a laurelled letter declaring the war to be at a close and bemoaning that he was too old to go on any further and repeat the conquests of Alexander the Great.[46] Since Charax was a de facto independent kingdom whose connections to Palmyra were described above, Trajan's bid for the Persian Gulf may have coincided with Palmyrene interests in the region.[57] Another hypothesis is that the rulers of Charax had expansionist designs on Parthian Babylon, giving them a rationale for alliance with Trajan.[58] The Parthian summer capital of Susa was apparently also occupied by the Romans.[59]