How can tell that the spread of Islam was not always peaceful?

Answer 1

What has to be clarified from the onset is that the "Spread of Islam" is a nebulous term. It refers to both the religion of Islam which expanded over more and more territory as time passed AND it refers to the Islamic Caliphates or Empires led by Muslim leaders (the Islamic equivalent of Christendom). However, the expansion of the religion is tied up with the expansion of the Islamic Caliphate, since when countries were annexed to the Caliphates, many citizens did convert either because they saw the Truth of the religion or to gain from the tax, work-related, and legal benefits that came from conversion.

To address the question, though, we know that the "Spread of Islam", especially as refers to the expansion of the Caliphates, and to a much lesser degree to the religion, occurred by way of violent conquest or subjugation. No country just opens its doors to conquerors and in many cases, we have the writings from Kings, Knights, and Soldiers attesting to their fear of conquest. Conversely, Muslims are very proud of their conquest of the Byzantine Empire and the "liberation" of its territories under General Khaled ibn Waleed and the destruction of the Sassanid Zoroastrian Persian Empire under General Sa'ad ibn Waqas. Some particularly useful examples, due how thoroughly they have been researched are: the Muslim Conquest of Iberia, the Mughal Conquest of central India, and Ottoman expansion into the Balkans.

It is worth noting that not all places that were incorporated into Islamic Empires required violence to conquer. In many places in North Africa, the Amazigh (Berber) Tribes were content to join with the Arab Muslim conquerors and unify their countries for a share in the booty from the next victim who would stand and fight. The Amazigh would get their prize when Spain was conquered.

Some Particular Examples of Violent Islamic Conquest:

Islamic Conquest of Persia

The Conquest of Persia is a clear example of the violence of Islamic conquest. After Mohammed's death several Arab Muslim tribes began to attack and pillage Sassanid Persian controlled cities in Mesopotamia. King Yazdagerd III of Sassanian Persia asked if the Arabs, led by Sa'ad ibn Abi Waqqas, would be willing to agree to a peace treaty where most of Mesopotamia would be under Arab control in return for no more pillaging. Waqas replied that the only way that there could be peace between the Arabs and the Sassanid Persians was if (a) the Persians all converted to Islam; or (b) the Sassanid Persian government paid a collective jizya tribute to the Muslims as a symbol of Muslim ascendance. Yazdagerd III refused both of these options and engaged the Arab Muslims. He was routed by the Arab Muslims in the Battles of Qadisiyyah and Nahavand, leading to the fall of Persia.

Islamic Conquest of Spain

There was a dynastic dispute in the Christian Visigothic Kingdom of Spain. Roderic was opposed by Akhila II. Roderic had a greater percentage of the nobility on his side, so Akhila II reached out to the Amazigh Muslims in Morocco for assistance in putting Roderic down. The Amazigh, led by Tariq ibn Ziyad, saw this as their opportunity, because without Spanish help, crossing the Strait of Gibraltar would be perilous. As soon as the Amazigh crossed in 711 C.E., they trounced Roderic in the Battle of Guadalete, killing him. They then advanced directly on Toledo, the Visigothic Capital, and imprisoned Akhila II after they conquered it. By 718 C.E. most of Spain was under Umayyad Control. The Muslims tried to expand into France, but were effectively prevented by Charles Martel.

Two centuries later, Almanzor was one of the most famous rulers of Andalucia (Islamic Spain), even if he was not a proper Caliph of the Umayyad Caliphate of Cordoba (also known as Andalucia). Almanzor is remembered historically in Spain precisely for punitive raids and devastating the various cities throughout northern Spain. He conducted over 60 such operations which destroyed Leon (984), Barcelona (985) Santiago de Compostela (997) Pamplona (999) and San Millan de la Cogolla (1002) along with other emblematic cities in the northern Christian Kingdoms. Almanzor fervently believed that he was acting in full compliance with the requirement of jihad (regardless of whether he was or not) and it is said that he would collect the dust and blood that stained his clothes during these anti-Christian incursions so that he could be interred with them. The argument that the Muslim invasion and conquest of Spain was peaceful is incorrect.

Islamic Conquest of India

The first major Islamic conquest of India by the Mughal dynasty is known as the Mughal-Rajput War of 1558-1578. Aurangzeb tried to expand this with the Mughal-Maratha Wars of 1681-1707. There is extensive documentation of these wars from both sides.

Islamic Conquest of the Balkans

The Ottoman wars with the Byzantine Empire (mostly in Asia Minor) raged for nearly two centuries, followed by wars with Bulgaria, Serbia, the Hungarian Empire, and the Austrian Empire, all of which continued from the fall of Thrace in 1380 to Siege of Vienna in 1529. We have detailed accounts from both Ottoman and European sources as to the disposition of forces in these campaigns, where the battles took place, what their outcomes were, and we also have phyisical evidence (such as weapons) that corroborate the story.