Columbia subject headings:
Harvard subject headings:
Abū Bakr, Caliph, -634.
Caliphs--Biography.
Islamic Empire--History, Military.
Islamic Empire--History--622-661.
Princeton subject headings:
Abū Bakr, Caliph, -634.
Caliphs--Biography.
Islamic Empire--History, Military.
Islamic Empire--History--622-661.