Anabasis (1.1.6-1.1.10)

Lastly, as regards his Greek force, he proceeded to collect it with the utmost secrecy, so that he might take the King as completely unprepared as possible.

The people of Miletus also were planning to do the very same thing, namely, to go over to Cyrus, but Tissaphernes, finding out about it in time, put some of them to death and banished others. Cyrus thereupon took the exiles under his protection, collected an army, and laid siege to Miletus both by land and by sea, and endeavoured to restore the exiles to their city; and this, again, made him another pretext for gathering an army.

Meanwhile he sent to the King and urged, on the ground that he was his brother, that these Ionian cities should be given to him instead of remaining under the rule of Tissaphernes, and his mother co-operated with him in this. The result was that the King failed to perceive the plot against himself, but believed that Cyrus was spending money on his troops because he was at war with Tissaphernes. Consequently he was not at all displeased at their being at war, the less so because Cyrus regularly remitted to the King the tribute which came in from the cities he chanced to have that belonged to Tissaphernes.