Expanding the Empire
Genghis Khan died in 1227, by which time the Mongol Empire ruled from the Pacific Ocean to the Caspian Sea — an empire twice the size of the Roman Empire and Muslim Caliphate. Genghis named his third son, the charismatic Ögedei, as his heir. The regency was originally held by Ögedei's younger brother Tolui until Ögedei's formal election at the kurultai in 1229.
Central Asian Expansion
Genghis Khan forged the initial Mongol Empire in Central Asia, starting with the unification of the Mongol and Turkic confederations such as Merkits, Tartars, Mongols, and Uighurs. He then continued expansion of the empire via conquest of the Kara-Khitan and the Khwarazmian dynasty. Large areas of Islamic Central Asia and northeastern Iran were seriously depopulated, as every city or town that resisted the Mongols was subject to destruction.
West Asian Expansion
The Mongols conquered, either by force or voluntary submission, the areas today known as Iran, Iraq, Syria, Caucasus and parts of Turkey, with further Mongol raids reaching southwards as far as Gaza into the Palestine region in 1260 and 1300. The major battles were the Siege of Baghdad (1258), when the Mongols sacked the city which for 500 years had been the center of Islamic power; and the Battle of Ain Jalut in 1260, when the Muslim Egyptian Mamluks were for the first time able to stop the Mongol advance at Ain Jalut in the southern part of the Galilee. One thousand northern Chinese engineer squads accompanied the Mongol Khan Hulagu during his conquest of the Middle East. The Mongols were never able to expand farther than the Middle East due to a combination of political and environmental factors, such as lack of sufficient grazing room for their horses.
East Asian Expansion
Genghis Khan and his descendants launched numerous invasions of China, subjugating the Western Xia in 1209 before destroying them in 1227, defeating the Jin dynasty in 1234, and defeating the Song Dynasty in 1279. They also destroyed the Tibetan Kingdom of Dali in 1253, forced Korea to become a vassal through an invasion of Korea, but failed in their attempts to invade Japan. The Mongols greatest triumph was when Kublai Khan established the Yuan Dynasty in China in 1271, though this dynasty was eventually overthrown in 1368 by the native Han Chinese, who launched their own Ming Dynasty. The Mongols also invaded Sakhalin Island between 1264 and 1308.
Southeast Asian
Kublai Khan's Yuan Dynasty invaded Burma in 1277, 1283 and 1287, resulting in Burma's capitulation and the disintegration of the Pagan Kingdom. The invasions of Vietnam and Java resulted in defeat for the Mongols, although much of South Asia agreed to pay tribute in order to avoid further bloodshed.
European Expansion
Historians regard the Mongol raids and invasions as some of the deadliest conflicts in human history up through that period. The Mongols brought terror to Europe on a scale not seen again until the twentieth century. The Mongol invasions induced population displacement on a scale never seen before, particularly in Central Asia and Eastern Europe. The impending arrival of the Mongol hordes spread terror and panic. The Mongols invaded and destroyed Volga Bulgaria and Kievan Rus', before invading Poland, Hungary and Bulgaria, and others. Over the course of three years (1237–1240), the Mongols destroyed and annihilated all of the major cities of Eastern Europe with the exceptions of Novgorod and Pskov.
Map above shows the expansion and conquest during the Mongolian Empire
A documentary by BBC on the secrets history of the Genghis Khan and how he successors were able to continue the empire in supreme power.
The image above shows a 1855 map of Asia under the Mongol Empire created by Karl Spruner von Merz.