632年6月8日，穆罕默德在麦地那病逝。穆罕默德生前没有指定继承人，部落酋长们认为，他们对穆罕默德的服从，已经随着先知的去世而告终结。于是，各部落停止进贡，各行其是。这一变故在伊斯兰教历史上视作“叛教”，引起了一系列有计划的战争。为了让贝都因人继续效忠于麦地那，最后经各派协商，同意由穆罕默德的岳父阿布·伯克尔出任先知在世间的代表，称为哈里发（khalīfat Rasūl Allah，意为真主使者的继承人）。正是在他的领导下，叛教的部落重新回到了信徒的行列，阿拉伯半岛恢复了统一，阿拉伯人开始了最早的对外扩张。
634年，奥马尔一世（634—644年在位）当选第二任哈里发，自称为“真主使者的继承人的继承人”，从此开始担任了穆斯林军队的总司令。他首先率领贝都因军队进入叙利亚，击败拜占廷，636年攻陷大马士革，两年后进军耶路撒冷地区；继而分兵东西两路，展开全面进攻。东线接连攻破波斯军队防线，占领了伊拉克和伊朗大部地区；西线连败拜占廷，占领巴勒斯坦和埃及。
第三任哈里发奥斯曼在位时，帝国扩张战争更是势不可挡。东线阿拉伯军队乘胜追击战败的波斯萨珊王朝，进军呼罗珊。651年，萨珊王朝灭亡，阿拉伯帝国夺得呼罗珊、亚美尼亚和阿塞拜疆等地。帝国西线大军则攻入北非利比亚的昔兰尼加。
正当对外扩张战争势如破竹时，帝国内部发生分裂。以阿里·伊本·艾比·塔里卜为首的哈希姆家族对出身于倭马亚家族的奥斯曼出任哈里发的合法性提出质疑，并组建起什叶派，与奥斯曼所奉行的逊尼派相对立。穆斯林内部纷争由此开端。656年，奥斯曼被刺，阿里继任哈里发。但以叙利亚总督穆阿维叶为首的倭马亚家族拒不承认阿里政权。双方数次火拼，未分胜负，僵持不下。不久，什叶派内部又出现分裂，部分对阿里不满的下层穆斯林组建了一个军事民主派——哈瓦立及派。661年，该派刺杀阿里，神权共和时代结束。
最初的四大哈里发由穆斯林公社选举产生，历史上将这4位哈里发统治时期称之为神权共和时期，又称四大哈里发时期。阿拉伯史学家把神权共和时期的四位哈里发叫做“拉什顿”(al-Rāshidūn 正统派)。这一时期，帝国版图迅速扩张，为阿拉伯帝国奠定了基础。
在此期间，历任哈里发利用北方拜占庭、波斯两大帝国长期争战的有利时机，以及新的伊斯兰教信仰带来的团结和力量，和他们自身充分的沙漠作战经验（特别是利用“沙漠之舟”骆驼，而不是像北部的邻居那样骑马作战），以“圣战”为名进行大规模的武力扩张。
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.