Complex sentence: The Muslims suffered another major defeat in the Battle of the Defile in 731, in which they lost some 20,000--30,000 men, crippling the native Khurasani Arab army and necessitating the transfer of new troops from Iraq.
Split into simpler sentences: The Muslims suffered another major defeat in the Battle of the Defile in 731, in which they lost some 20,000--30,000 men. The loss of a major portion of the native Khurasani Arab army necessitated the transfer of new troops from Iraq.