Columbia subject headings:
Harvard subject headings:
Islamic Empire--History--622-661.
Islamic Empire--Politics and government.
ʻUmar ibn al-Khaṭṭāb, Caliph, -644.
Princeton subject headings:
Islam and state.
Political science--Islamic Empire--History.
ʻUmar ibn al-Khaṭṭāb, Caliph, -644.