But that night Xenophon made up his mind to do what he could to save his countrymen.  The only hope was in some one taking the lead, and, as the Greeks had been true to their oaths throughout the whole march, he believed the gods would help them.  So he called the chief of the officers still remaining together, and put them in mind that they might still hope.  They were so much stronger and braver than the Persians, that if only they did not lose heart and separate, they could beat off almost any attack.  As to provisions, they would seize them, and the rivers which they could not cross should be their guides, for they would track them up into the hills, where they would become shallow.  Only every soldier must swear to assist in keeping up obedience, and then they would show Artaxerxes that, though he had seized Clearchus, they had ten thousand as good as he.  The army listened, recovered hope and spirit, swore to all he asked, and one of the most wonderful marches in the world began.  Cheirisophus, the eldest officer, a Spartan, took the command in the centre; Xenophon, as one of the youngest, was in the rear.  They crossed the Zab, their first barrier, and then went upwards along the banks of the Tigris.  The Persians hovered about them, and always attacked them every morning.  Then the Greeks halted under any shelter near at hand, and fought them till towards evening.  They were sure to fall back, as they were afraid to sleep near the Greeks, for fear of a night attack.  Then the Greeks marched on for a good distance before halting to sup and sleep, and were able again to make a little way in the morning before the enemy attacked them again.