There was, however, one danger to be guarded against in ordering the army to march toward the capital, and that was, that Ptolemy, while under Cæsar's influence, might open communications with the officers, and so obtain command of its movements, and thwart all the conspirators' designs. To prevent this, it was arranged between Pothinus and Achillas that the latter should make his escape from Alexandria, proceed immediately to the camp at Pelusium, resume the command of the troops there, and conduct them himself to the capital; and that in all these operations, and also subsequently on his arrival, he should obey no orders unless they came to him through Pothinus himself. Although sentinels and guards were probably stationed at the gates and avenues leading from the city, Achillas contrived to effect his escape and to join the army. He placed himself at the head of the forces, and commenced his march toward the capital. Pothinus remained all the time within the city as a spy, pretending to acquiesce in Cæsar's decision, and to be on friendly terms with him, but really plotting for his overthrow, and obtaining all the information which his position enabled him to command, in order that he might co-operate with the army and Achillas when they should arrive. All these things were done with the utmost secrecy, and so cunning and adroit were the conspirators in forming and executing their plots, that Cæsar seems to have had no knowledge of the measures which his enemies were taking, until he suddenly heard that the main body of Ptolemy's army was approaching the city, at least twenty thousand strong. In the mean time, however, the forces which he had sent for from Syria had not arrived, and no alternative was left but to defend the capital and himself as well as he could with the very small force which he had at his disposal. He determined, however, first, to try the effect of orders sent out in Ptolemy's name to forbid the approach of the army to the city. Two officers were accordingly intrusted with these orders, and sent out to communicate them to Achillas. The names of these officers were Dioscorides and Serapion.