Who were the mongolians and what did they do?

The Mongols were a nomadic race of people who are from Mongolia as well as China’s Inner Mongolia Autonomous Region. They were expert riders, hunters and archers.  Originally, they were group of independent tribes until they were united by Genghis Khan and later embarked on a campaign of conquest.  Due to their tactics and speed on horseback, they were easily able to defeat other armies which led to the establishment of the Mongol Empire.  It was composed of Mongolia, North China, Turkmenistan, Afghanistan and Northern India.  After the Genghis Khan his descendants conquered more countries but eventually their power declined and some of the Mongols were absorbed by the countries they had conquered.

What war are you specifically talking about?