234 climate and geography: The Islamic World by inundating the earth in the valley with water and by laying down a new deposit of rich topsoil As a result, whereas most agriculture in the medieval world might return seven grains for each seed planted, fields in Egypt routinely returned 20:1, making Egypt the most agriculturally productive area in the world Mesopotamia Mesopotamia (modern-day Iraq and Kuwait) became the political center of Islamic civilization The city of Baghdad (meaning “gift of God”) was founded in 762 as the new capital of the caliphate, the central government nominally in charge of the whole Islamic Empire The name Mesopotamia comes from the ancient Greek meaning “between the rivers.” The “rivers” are the Tigris and Euphrates Like the Nile in Egypt, they carried water to an essentially desert region to make very productive agriculture possible and enabled the emergence of one of the world’s first civilizations (Sumer and Akkad) in antiquity But unlike agriculture in Egypt around the Nile, agriculture in Mesopotamia depended on a very complex and delicate system of canals and irrigation, and the rivers’ frequent flooding was destructive rather than beneficial The irrigation system had always used waterwheels and other machinery to regulate the flow of water and generate mechanical power, but these systems were considerably improved by engineers such as al-Jazari (fl 1206) His machines tended to replace human and animal power with flowing water and used devices such as cranks; he was one of the first engineers to realize how to convert circular motion into reciprocal motion (such as the up-and-down motion of a pump) The high level of technology achieved was a real advance on Greek models inherited from antiquity and far more sophisticated than contemporary European technology Iran and I nner Asia The Plateau of Iran lies between the Persian Gulf and the Caspian Sea It has been thrust up by the subduction of the Arabian underneath the Eurasian tectonic plate The Zagros Mountains run along the southern edge of the plateau and serve as the border with Mesopotamia This range has peaks above 14,764 feet Most of the land is arid, with the only wellwatered plain lying at the southern end of the Caspian Sea Hence the country is much better suited to pasturage than agriculture Accordingly, the national way of life in Iran was horse breeding, and the country’s military might consisted of its well-mounted and highly trained cavalry The Caspian Sea is the world’s largest inland body of water, with an area of 143,244 square kilometers Fed at its northern end by the Volga River, the area around the Volga delta is fresh water, but much of the Caspian is highly saline To the east of the Caspian Sea and the north of the Plateau of Iran lie the inner Asian steppes This vast area of grassland supported many peoples who lived from horse breeding and who consequently made highly effective cavalry and were repeatedly able to successfully raid and conquer more settled peoples in Europe, India, and China The stirrup, which makes riding and especially fighting from a horse much easier, was invented in this area at the very beginning of the Middle Ages To some degree this development triggered an outward migration from inner Asia that eventually overwhelmed the Roman Empire The first great conquerors from inner Asia were the Iranians themselves, who in antiquity gained an empire that stretched as far as Egypt in Africa and Thrace in Europe They were followed by Huns and Turks and eventually by Mongols from the desert regions somewhat farther east The latter two groups in general converted to Islam In the Middle Ages the population of inner Asia was made up of various Turkic and Iranian peoples Originally the area was marked by religious diversity, with many nations following traditional religions and others embracing ancient world religions, such as Manichaeism and Buddhism But by the end of the Middle Ages the entire region was almost uniformly Islamic The nature of the inner Asian steppes—vast expanses of grassland with no major rivers or settled populations to impede travel, made the area a major trade route between East and West This was the so-called Silk Road, actually a network of caravan routes by which silk, porcelain, and other precious items were shipped from China to Europe, mostly in exchange for gold and silver Without actual roads or other infrastructure to support trade, the caravans consisted of large strings of horses or camels carrying relatively small loads on their backs without the aid of wheeled vehicles such as carts or wagons, which could not easily be dragged through the tall grasses The caravan routes passed either north of the Caspian Sea to end in ports on the Black Sea or to the south to end in cities on or near the Mediterranean coast, such as Damascus or Beirut India and the I ndian Ocean Islamic military operations to conquer India were attempted shortly after the conquest of the Sassanid Empire (651), but the greater part of the Indian subcontinent did not fall to Islamic rule until the establishment of the Mughal Empire in India in the 15th and 16th centuries The Muslim heartland in India has always been the Sind (modern-day Pakistan), the valley of the Indus River This was the host to another ancient civilization, as were the Nile and Tigris-Euphrates valleys Islamic control of India together with central Asia produced a monopoly over the two major east–west trade