But an attack on the Persian host in Asia Minor did not turn out to be at once feasible. A sudden danger at home obliged Heraclius to delay his crusade. The Avars concentrated their ravages on Thrace, and their hordes rode up almost to the gates of Constantinople. It was necessary at all costs to free the city from the danger of attack in the rear before the army crossed over into Asia. [Sidenote: Treachery of the Avars.] Accordingly the emperor sent to offer a subsidy to the Chagan of the Avars if he would withdraw beyond the Danube. The Chagan proposed a conference at Heraclea, forty miles west of Constantinople, the point to which he had advanced his army. Heraclius consented to the meeting, and rode out in royal state, with all his court. But the faithless Avar was meditating treachery. He concealed troops of his horsemen in the hills, with the object of waylaying Heraclius on his way to Heraclea, and of holding him to ransom. The emperor was warned just in time to escape from the ambush. Throwing off his long purple robe, and tucking his diadem under his arm, he rode hard for Constantinople, with the Avars close at his heels. Many of his court, and thousands of the Thracian peasantry, who had turned out to witness the meeting, fell into the hands of the enemy. Heraclius had just time to order the gates to be closed before the pursuers swept through the suburbs, and up to the walls. In spite of this piece of abominable treachery, the emperor was still fain to conclude a peace with the Avars, as an absolutely necessary preliminary before attacking the Persian. In 620 a peace of some sort was patched up, in return for a payment of money, but even then Heraclius was not able to start on his projected campaign. Some desultory Persian attacks on Constantinople, and notably an attempt to build a fleet at Chalcedon, and cross the strait, had first to be frustrated. It was not till 622 that the emperor was finally enabled to take the offensive. But all preparations being complete, after solemnly keeping the Lenten Fast, and receiving the benediction of the Church for himself and his army, he set sail for Asia on Easter Day. He left his young son, Heraclius Constantinus, regent in his stead, under the charge of the patriarch Sergius and the patrician Bonus, the commander of the garrison of Constantinople.