The Mongols were nomadic warriors who conquered from the steppe grasslands of Central Asia and Mongolia, and embarked on a campaign of worldwide conquest in the 13th century, following their unification by Genghis Khan. Despite early Mongol conquests being based on speed and superior tactics against armies in open fields, they eventually had to face fortified cities. A change in Mongol strategy was necessary due to their smaller numbers; it wasn't wise in the long term to just sack every city they conquered, as it would leave little to rule and few resources to benefit from. During their conquest of China, they learned about siege warfare that would greatly benefit their future conquests under different Khans through Central Asia, the Middle East, and parts of Russia and Europe.
In many ways, the Mongols were the first conquerers to use terror as a psychological weapon in their conquests. If a city surrendered, its people were spared and came under the Mongol “yoke” (control). If they resisted, the city was made an example of and its entire population was killed, including civilians. While this may seem harsh, it did ensure a period known as the Pax Mongolica during the height of the Silk Road. Put simply, if traders and merchants or people under Mongol protection were harmed, the Mongol hordes would come and kill you in much the same way as above. It was brutal but effective, and brought enforced peace to a chaotic reign for many decades. They also adapted to enemy defenses, even using diseased bodies via catapult as an early form of biological warfare. This might have even helped bring the Black Plague to Europe.