632年6月8日，穆罕默德在麥地那病逝。穆罕默德生前沒有指定繼承人，部落酋長們認為，他們對穆罕默德的服從，已經隨着先知的去世而告終結。於是，各部落停止進貢，各行其是。這一變故在伊斯蘭教歷史上視作“叛教”，引起了一係列有計劃的戰爭。為了讓貝都因人繼續效忠於麥地那，最後經各派協商，同意由穆罕默德的嶽父阿布·伯剋爾出任先知在世間的代表，稱為哈裏發（khalīfat Rasūl Allah，意為真主使者的繼承人）。正是在他的領導下，叛教的部落重新回到了信徒的行列，阿拉伯半島恢復了統一，阿拉伯人開始了最早的對外擴張。
634年，奧馬爾一世（634—644年在位）當選第二任哈裏發，自稱為“真主使者的繼承人的繼承人”，從此開始擔任了穆斯林軍隊的總司令。他首先率領貝都因軍隊進入敘利亞，擊敗拜占廷，636年攻陷大馬士革，兩年後進軍耶路撒冷地區；繼而分兵東西兩路，展開全面進攻。東綫接連攻破波斯軍隊防綫，占領了伊拉剋和伊朗大部地區；西綫連敗拜占廷，占領巴勒斯坦和埃及。
第三任哈裏發奧斯曼在位時，帝國擴張戰爭更是勢不可擋。東綫阿拉伯軍隊乘勝追擊戰敗的波斯薩珊王朝，進軍呼羅珊。651年，薩珊王朝滅亡，阿拉伯帝國奪得呼羅珊、亞美尼亞和阿塞拜疆等地。帝國西綫大軍則攻入北非利比亞的昔蘭尼加。
正當對外擴張戰爭勢如破竹時，帝國內部發生分裂。以阿裏·伊本·艾比·塔裏卜為首的哈希姆傢族對出身於倭馬亞傢族的奧斯曼出任哈裏發的合法性提出質疑，並組建起什葉派，與奧斯曼所奉行的遜尼派相對立。穆斯林內部紛爭由此開端。656年，奧斯曼被刺，阿裏繼任哈裏發。但以敘利亞總督穆阿維葉為首的倭馬亞傢族拒不承認阿裏政權。雙方數次火拼，未分勝負，僵持不下。不久，什葉派內部又出現分裂，部分對阿裏不滿的下層穆斯林組建了一個軍事民主派——哈瓦立及派。661年，該派刺殺阿裏，神權共和時代結束。
最初的四大哈裏發由穆斯林公社選舉産生，歷史上將這4位哈裏發統治時期稱之為神權共和時期，又稱四大哈裏發時期。阿拉伯史學家把神權共和時期的四位哈裏發叫做“拉什頓”(al-Rāshidūn 正統派)。這一時期，帝國版圖迅速擴張，為阿拉伯帝國奠定了基礎。
在此期間，歷任哈裏發利用北方拜占庭、波斯兩大帝國長期爭戰的有利時機，以及新的伊斯蘭教信仰帶來的團结和力量，和他們自身充分的沙漠作戰經驗（特別是利用“沙漠之舟”駱駝，而不是像北部的鄰居那樣騎馬作戰），以“聖戰”為名進行大規模的武力擴張。
Abu Bakr, the first successor of Muhammad, according to Sunni beliefs, nominated Umar as his successor on his deathbed, and there was consensus in the Muslim community to his choice. Umar Ibn Khattab, the second caliph, was killed by a slave. His successor, Uthman Ibn Affan, was elected by a council of electors (Majlis), but was soon perceived by some to be ruling as a "king" rather than an elected leader. Uthman was killed by members of a disaffected group. Ali then took control but was not universally accepted as caliph by the governors of Egypt, and later by some of his own guard. He faced two major rebellions and was assassinated after a tumultuous rule of only five years. This period is known as the Fitna, or the first Islamic civil war. Under the Rashidun each region (Sultanate, Wilayah, or Emirate) of the Caliphate had its own governor (Sultan, Wāli or Emir).
Muawiyah, a relative of Uthman, and governor (Wali) of Syria became one of Ali's challengers, and after Ali's death, managed to overcome the other claimants to the Caliphate. Muawiyah transformed the caliphate into a hereditary office, thus founding the Umayyad dynasty.
In areas which were previously under Sassanid Persian or Byzantine rule, the Caliphs lowered taxes, provided greater local autonomy, greater religious freedom for Jews, indigenous Christians, and brought peace to peoples demoralized and disaffected by the casualties and heavy taxation that resulted from the decades of Byzantine-Persian warfare.