While Nadir was employed at the siege of Herat the Persian nobles at Ispahan persuaded the weak Tamasp to place himself at the head of an army and march against the Turks, who were again assembling on the frontier. The reverses which the arms of that nation had sustained in Persia had caused a revolution at Constantinople, where the janizaries had first murdered the vizier, and afterward dethroned Achmet, and placed his nephew, Mahmud, upon the throne. To this Prince Nadir had sent an envoy, demanding that the Turks should evacuate the province of Azerbaijan; and Shah Tamasp had sent another with what a Persian historian indignantly terms "a sweet-scented letter of congratulation" upon his elevation to the throne. Before the result of the mission sent by Nadir could be known, Tamasp had marched to besiege Erivan, had retreated from before that fortress, been defeated by a Turkish army, and had lost in one month all that the genius and valor of his general had gained during the preceding season. To render the effects of his weakness complete, the alarmed monarch had agreed to a peace, by which he abandoned the whole of the country beyond the Araxes to the Turks, and ceded five districts of the province of Kirmanshahan to Achmet, the reigning pacha of Bagdad, by whom this treaty was negotiated. The disgrace of this engagement was aggravated by its containing no stipulation for the release of the Persians who had been made prisoners during the war. The moment that Nadir received accounts of the peace it seems to have occurred to his mind that it afforded an excellent pretext for the consummation of those projects he had so long cherished: but, although bold and impatient, he was compelled to proceed with caution to the extinction of a race of kings to whom obedience had become a habit, and who were at that moment represented by a prince who, though weak and despicable, was endeared to many of his subjects by his misfortunes. His first step was to issue a proclamation, in which he inveighed with bitterness against a treaty which bounded the great empire of Persia by the river Araxes, and left many of the inhabitants of that kingdom prisoners in the hands of cruel enemies. "Such a treaty," he said, "is contrary to the will of Heaven: and the angels who guard the tomb of the holy Ali call aloud for the deliverance of his followers from the bondage in which they are now held by vile heretics."