Why were Muslim armies able to expand the land under their control so rapidly?

Answer 1

The early Caliphates (or Islamic Empires) expanded due to a number of factors. It is notable that although there are a few occurrences of Muslims carrying out the "Convert or Die" scenario, this is not the most common reason for Islam's expansion exclusively in terms of territory.

1) Arabia - What's that?: Prior to Mohammed, Arabia had been seen as a cultural and political backwater barely worth notice and therefore Mohammed's consolidation of the Arab Tribes was not seriously considered by the Byzantine and Sassanian Empires. This lead to a very porous southern defense and miscalculation of the Islamic Armies.

2) Byzantine-Sassanian Wars: Since the Byzantines and Sassanians considered each other the only real civilizations in the region, they fought each other numerous times in the early 600s C.E. over control of what is today East Turkey and Iraq. These were intense religious wars (Christian vs. Zoroastrian) and greatly weakened both empires at just the wrong time.

3) Tribal Command Structure: Arab Tribes and warriors owed their loyalty to their Sheikhs and leaders without fault because the Sheikh provided each person with all of the amenities and protection necessary to live a full life. Whenever the Arabs went to war, the Sheikhs could count on the ability to muster their tribesmen as soldiers without issue.

4) Support of the Locals: Since the Byzantine Empire and Sassanian Empire were engaged in religious zealotry, it stands to reason that they did not tolerate their religious minorities very well. The Muslims professed to (and did) treat minorities far better than either of these Empires. This led to religious minorities (especially Jews) from within the conquered territories helping Muslims to secure outposts and positions, freeing the Muslims to continue to spread the religion.

5) Citizenship by Conversion: The Islamic Empire was interesting in that being the leader or a person of import in the Empire was not contingent on ethnicity or place of birth as was the case of most prior empires. All a person had to do to gain power and prestige was to convert to Islam. This simple integration mechanism resulted in massive number of Persians, Berbers, and Turks converting to Islam. In fact, the furthest expansions of Islam (Spain, India, West China, South Russia, the Balkans) were all accomplished by non-Arabs (respectively Berbers, Persians/Mughals, Turkmen, Azeris, and Ottomans).

6) Warfare: The leaders charged with expanding the influence of Islam were thoughtful and diligent military leaders. Caliph 'Omar, who conquered much of the Levant, Egypt, Persia, and Iraq was well-known as a strategist and a fighter. It was this knowledge of warfare that allowed Muslim forces to expand the territory's influence.

7) Universality: Islam, as a religion, claims salvation for all people in all places, making the religion very attractive, especially to people born into low castes and bad economic positions.

Answer 2

The Muslim armies were very capable over habitable lands, which explains their quick conquest of the Northern Middle East and of Mediterranean Africa. However, both the Sahara Desert and the Mediterranean Sea were difficult for the armies to cross. The Strait of Gibraltar was only crossed with rogue Visigothic assistance and the Sahara was only crossed by merchants, not armies, leading to a slower spread of Islam in Sub-Saharan Africa. Similarly, Constantinople's geographic position, being protected by three sides by water and the fourth by a high wall, prevented its inclusion in Seljuk and Ottoman Empires for nearly 350 years.

As concerns Islam's expansion through peaceful means, this used to occur primarily through trade routes. As a result, Islam began to flourish along the East African coast (such as Zanzibar, Tanzania) and the Indonesian Islands, but did not penetrate into the mainland regions such as west Tanzania or Burma/Thailand.

Currently, Islam is spreading primarily through Immigration of Muslims to non-Muslim majority countries due to economic circumstances.

Answer 3

North Africa is relatively flat, so this made it much easier for Arabs to swiftly conquer it.