Final answer:
Umar ibn al-Khattab, the second Rashidun Caliph, was responsible for the conquests of Mesopotamia, Palestine, Syria, Persia, and Egypt during his caliphate from 634-644 CE.
Step-by-step explanation:
The caliph who conquered Mesopotamia, Palestine, Syria, Persia, and Egypt and spread Islam to these lands was Umar ibn al-Khattab. His reign as the second of the “rightly guided” or Rashidun Caliphs (634-644 CE) marked a significant period of expansion for the Islamic Caliphate. Under Umar's leadership, the caliphate's military campaigns were dramatically extended, resulting in the occupation of the eastern parts of the Byzantine Empire, which included territories like Syria, Egypt, and Anatolia, as well as the entirety of the Persian Empire in the 640s. Umar's greatest achievement was not only military; he also established an administrative structure to manage the vast empire, introducing the diwan, a state bureaucracy for tax collection, public safety, and administering the Sharia law.