The eastern boundaries of Iran are the geographical manifestation of a long political process of two powers - Iran and Britain - acting in rivalry against each other. Historically, Iran's eastern frontiers had, until the mid-19th century, never had a clearly defined geographical limit. These frontiers, depending on the strength and weakness of the central authorities in Iran, moved back and forth across central Asia. They sometimes included places as far east as Kabul and what is now northern India, while such places as Bokhara, Samarqand, Merv, Khivah, Balkh and Herat featured prominently in Persian literature as traditional Iranian cities.