Mongol military tactics and organization enabled the Mongol Empire to conquer nearly all of continental Asia, along with parts of the Middle East and Eastern Europe. That system was founded originally on the expansion of the nomadic lifestyle of the Mongols. Other elements were invented by Genghis Khan, his generals, and his successors. Technologies useful for attacking fortifications were adapted from other cultures. Foreign technical experts were integrated into the command structure. In many cases, the Mongols defeated significantly larger armies. Transfer of troops between units was forbidden. The leaders on each level had significant license to execute their orders in the way they considered best. This command structure was highly flexible and allowed the Mongol army to attack en masse, divide into smaller groups to encircle their enemies and either lead them into an ambush or divide into small groups of roughly 10 to mop up a fleeing and broken army. Although they fought as a unit, individual soldiers were responsible for their equipment, weapons, and up to five mounts. Their families and herds would accompany them on foreign expeditions. Standing above other units was an elite force called Kheshig. They functioned as imperial guard of the Mongol Empire as well as a training ground for potential young officers. Subutai, a powerful Mongol general, started his career in the Kheshig. Mobility Each Mongol soldier typically maintained 3 or 4 horses. Changing horses often allowed them to travel at high speed for days without stopping or wearing out the animals. When one horse became tired, the rider would dismount and rotate to another. Though the used mount would still have to travel, it would do so without the weight of the rider. Their ability to live off the land, and in extreme situations off their animals (mare's milk especially), made their armies far less dependent on the traditional logistical apparatus of agrarian armies. In some cases, as during the invasion of Hungary in early 1241, they covered up to per day, which was unheard of by other armies of the time. The mobility of individual soldiers made it possible to send them on successful scouting missions, gathering intelligence about routes and searching for terrain suited to the preferred combat tactics of the Mongols. During the Mongol invasion of Kievan Rus', they used frozen rivers as highways, and winter, the time of year usually off-limits for any major activity due to