Properly speaking, it was not an army at all, but rather a people in movement.

The grassy Steppes fed the flocks, and the flocks fed the warriors; and with such a simple commissariat system there was no necessity for keeping up communications with the point of departure.

Instead of diminishing in numbers, the horde constantly increased as it moved forwards.

The nomadic tribes which it encountered on its way, composed of men who found a home wherever they found pasture and drinking-water, required little persuasion to make them join the onward movement.

By means of this terrible instrument of conquest Genghis succeeded in creating a colossal Empire, stretching from the Carpathians to the eastern shores of Asia, and from the Arctic Ocean to the Himalayas.
Genghis was no mere ruthless destroyer; he was at the same time one of the greatest administrators the world has ever seen.