Early modern warfare was invented in China during the twelfth and thirteenth centuries. It was during those two centuries of brutal warfare between the Chinese Song dynasty, the Jurchen Jin dynasty, and the rising power of the Mongols that guns, grenades, rockets, and other incendiary weapons fueled by gunpowder became regular and widespread tools of war. These weapons were used extensively in siege and naval warfare by vast armies and navies, and gradually moved on to the open battlefield. Chinese soldiers were recruited, trained, and armed by the government, and organized into regularly ordered military units supplied by a bureaucratic logistics system, as indeed they had been for more than a thousand years. These troops were even housed in barracks and provided with regular medical care. The major sieges of the time revolved around cities with relatively low, thick walls, almost impenetrable to missiles, with circumferences measured in miles. True guns developed in this environment, and subsequently spread to the rest of Asia and the world.