At the height of their empire, the Mongols' wrath would be felt from East Asia to Western Europe, a dynasty which would live on well into the Twentieth Century. A true chronicle of their history must reconcile the horrors of the conquest with their skill in uniting the disparate parts of the empire from technology to culture.
What we now consider the Mongols were a collection of tribes emerging from disparate tribes of the steppes of Central Asia such as the Xiongu (beginning in 209 BCE) and the Khitai (making their presence known in the Fourth Century CE).
These nomadic, warlike tribes would develop a technique for shooting a target while riding on horseback which would make them formidable foes to settlements and kingdoms throughout the region.
These so-called barbarian hordes would occasionally consolidate into a more fearsome fighting force, only to be repelled (and in the case of the Xiongu against the Han Dynasty) nearly face extinction. And by the Second Century BCE, along with the Tartars, the Mongols would prove to be such an irritant to the Chinese, the Han emperor would both order their extermination as well as the construction of the Great Wall.
What we know as the 'true' Mongol Empire would begin with Temujin, born on the steppes in 1162. He was the son of a Borjigin chieftain, who would go to war with his regional rivals and quickly defeat them through spy craft and building an army based on merit and skill rather than blood ties. By 1206, his fighting force would subdue and absorb the western Naiman tribe, the Merkits in the north, and the Tanguts in the south, and that year, Temujin would declare himself 'Genghis Khan,' as one does.
The first thing that the universal leader of the Mongols would do is establish a unified code of laws, or the Yassa. The Yassa provided civil structure to the newly-built Empire, holding king and commoner to account and focusing on the dispensation of property, brides, and require civil or military services.
Under Genghis's Yassa, all citizens were granted religious freedom, as long as they maintained strict loyalty to Genghis above all. Religious leaders were free from both taxation and both civil and military service to the Empire.
Under Genghis' third son, Ogedei, the Khans would become patrons of houses of temples and houses of worship for the Taoists, Christians, Buddhists, and Muslims of Karakorum. Later, the Empire would fund Christian churches in China, Buddhist temples in Persia, and Muslim schools in Russia. It was a handy method of respecting local tradition while making it subordinate to the law of the Empire.
By the time of Genghis' grandson Batu's expansion into the Balkans in the 1240s, the Empire would become known as the Khanate of the Golden Horde. Batu would establish its capital in the city of Sarai on the Akhtuba River near modern-day Slitrennoye in Russia. By the 14th Century, theirs would be the largest contiguous land empire in history, covering between 11 and 12 million square miles.
In conquest mode, Mongol fighters rode light, moving rapidly and collecting what they needed on the way in order to build ladders, bridges, and siege engines. Each man would be responsible for securing or making his own bow for combat. Instead of settling in villages or cities, they would camp under hastily assembled stretched felt, wicker-reinforced shelters.
Then, they would lay siege to even the most fortified of cities using weapons collected from cultures across the empire, using both technology and the sheer force of their reputation.
And while not responsible for much in the way of art in culture, the way their empire was constructed allowed art, culture, and technology from disparate corners to spread from one end to another. For instance, when Hulegu Khan began his campaign against Baghdad in the 1250s, he would bring 1,000 Chinese catapult engineers (and their entire households), utilizing these men's know-how against Baghdad's walls. Later, Syrians familiar with counterweight catapults from the Iranian Khanate would go to China to assist the Yuan against the Sung in the south.
The Mongols were formidable, they were feared, and the only enemy which could disrupt the Empire would be the Empire itself. The main body of the Empire would fracture with the death of Mongke Khan in 1259. Mongke had no chosen successor, so his sons and relatives each decided to fill the position—all at once.
By 1271, civil war would fracture the Empire, splitting into four Khanates: the Golden Horde, descended from Batu, would dominate Russia and the Western Steppes. In Western Turkestan, the Muslim Chagatai Khanate (descended from Genghis' third son, Ogedei) would flex its strength for five centuries across Central Asia, along with parts of what are now modern Russia, China, and Afghanistan. The Ilkhanate would stretch from Iran to large swaths of Central Asia while Genghis' grandson, Kublai Khan, would destroy the Song Dynasty in China and install the Yuan Dynasty.
The Empire would get a second wind under Tamerlane who, between 1380 and 1400, would conquer the area between Iran, Khorasan, Harat, Baghdad, India, Azerbaijan, and Anatolia, effectively reunifying the disparate Khanates (for a time).
Today, the Empire may be gone, but the Mongols still endure, with Genghis' last ruling descendant, Alim Khan, living well into the 20th Century and governing Uzbekistan, and the remaining people surviving Soviet purges, living on with their own (disputed) independent country.