After the death of the Prophet Muhammad in 632 the armies of Islam swept out of the Arabian Peninsula. Within fifty years they had overrun all the territory conquered by Alexander the Great nine hundred years before. Within one hundred years their empire extended from Spain to India and from Egypt to the Caspian Sea in the north. The Caliphs then set about consolidating their empire, building a new capital of Baghdad in 762. This was strategically located on the fertile plain of the river Tigris, away from the reach of marauding armies, yet on the lucrative trading route of the Silk Road to China. The centre piece of the new capital was The House of Wisdom (Bayt al-Hikma), a combination of publishing house, library and research institute. This became the focus of a vigorous expansion of knowledge in all of the sciences including medicine.