Khorasan was the second largest province of the Sassanid Persian Empire. It stretched from what is now present day north eastern Iran, Afghanistan and Turkmenistan. Its capital was Balkh, now in present day northern Afghanistan. In 651 the mission of conquering Khurasan was assigned to Ahnaf ibn Qais. Ahnaf marched from Kufa and took a short and less frequent route via Rayy and Nishapur. Rayy was already in Muslim hands and Nishapur surrendered without resistance. From Nishapur Ahnaf marched to Herat which in now in present day southern Afghanistan. Herat was a fortified town, Siege of Herat lasted for few months before Herat surrendered. With the surrender of Herat, the whole of the southern Khurasan came under Muslim control. With Herat under his firm control, Ahnaf marched north direct