Hannibal, it is certain, could never have performed that wondrous march from the edge of Andalusia right up through Spain, over the Pyrenees, across France and beyond the Alps into the plains of Piedmont, where he fought his most artistic battle in 218 B.C., at the Trebia, had it not been for an organisation of spies and informers who prepared the way by ruse and diplomacy for the advance of his hordes. Of him Polybius writes: "For years before he undertook his campaign against Rome, he had sent his agents into Italy and they were observing everyone and everything. He charged them with transmitting to him exact and positive information regarding the fertility of the trans-Alpine plains and the valley of the Po, their populations, their military spirit and preparations and, above all, their disposition towards the government at Rome. There was nothing too large in promises that the Carthaginian was not ready to make in return for their support against the hated City." Cæsar too employed spies to the undoing of his adversaries in Egypt, in Gaul and also in Britain, and although in his Commentaries he records his employment of emissaries of this kind, history remains generally blank as to special details, leaving us to conclude that, like Napoleon, he relied mainly on the exigencies of the moment to produce the required information through the bribery of individuals in the opposite camp. In his early political career, especially during his tenure of the office of Pontifex Maximus, it seems clear that he then laid the leading lines, through the employment of many informers, of that vast political network of which he subsequently became the master, while his later association with Marcus Crassus, who mainly owed both wealth and power to the army of spies which he controlled, was in every way to Cæsar's advantage in respect of the means of procuring important information. Had he employed the services of a spy system on his attainment to supreme power, it is unlikely that he would have come to his destruction at the hands of a group of the best-known men in Rome, the fact leaving us to infer that he had ceased to use a secret service after the Civil War.