The war on the Scyths was dramatic and exciting, both sides acting in the spirit of chivalry. Crossing the Bosporus, Darius advanced through Thrace to the Danube which he spanned with a bridge. The Scyths adopted the favourite Russian plan of retreating into the interior, destroying the crops and hovering round the foe; they further led the Persians into the territories of their own enemies. This process at last wearied Darius; he sent a herald to challenge them to a straight contest or to become his vassals. The reply came that if Darius wished a conflict he had better outrage their ancestral tombs; as for slavery, they acknowledged only Zeus as their master. But the threat of slavery did its work. A detachment was sent to the Danube to induce the Ionian Greeks to strike for freedom by breaking down the bridge they were guarding, thus cutting off Darius' retreat. To the King himself a Scythian herald brought a present of a bird, a mouse, a frog and five arrows, implying that unless his army became one of the creatures it would perish by the arrows. The Scyths adopted guerilla tactics, leaving the Persians no rest by night and offering no battle by day. At last Darius began his retreat. One division of the Scythian horsemen reached the bridge before their foes, again asking the Ionians to destroy it. The Greeks pretended to consent, breaking down the Scythian end of it. Darius at last came to the place; to his dismay he found the bridge demolished. He bade an Egyptian Stentor summon Histiaeus, the Greek commandant, who brought up the fleet and saved the Persian host which retired into Asia. In 509 a second expedition was dispatched against Barca, a colony of Cyrene. The history of the latter is graphically described, the first king being Battus, the Stammerer, who founded it in obedience to the directions of Apollo. Cyrene was brought under Cambyses' sway by Arcesilaus who had been banished. He misinterpreted an oracle and cruelly killed his enemies in Barca. When he was assassinated in that town his mother Pheretima fled from the metropolis Cyrene to Aryandes, the Persian governor in Egypt. Backed by armed force she besieged Barca which resisted bravely for nine months; at the end of that term an agreement was made that Barca should pay tribute and remain unassailed as long as the ground remained firm on which the treaty was made. But the Persians had undermined the spot, covering planks of wood with a loose layer of earth. Breaking down the planks they rushed in and took the town, Pheretima exacting a horrible vengeance. Yet she herself died soon after, eaten of worms. "Thus," remarks the historian, "do men, by too severe vengeances, draw upon their own heads the divine wrath."