Prophet Muhammad has a unique place in world history. During his life time  he propounded a new faith, which for its time and age was progressive.  It struck at idolatry and  propounded the theme of one omnipotent god. After his death in 632 AD, a revolution of sorts took place. Inspired by the zeal of the prophet, Muslim armies spread out from Saudi Arabia to the far corners of the known world. Muhammad bin Qasim headed towards India and defeated the Hindu king Dabir. Muslim armies also spread out across North Africa and crossed the strait of Gibraltar and conquered Spain. They also moved into Asia Minor and subjugated Turkey and Eastern Europe. Within a span of 3 centuries the Muslim armies had conquered an area that now houses some 80 nations.

The Muslim invaders were fired by a zealous faith and had a belief to spread Islam. This they did not do by love and compassion, but by the sword. The conquered people had no choice but to convert to Islam or be  beheaded. Islam thus spread like a wild fire. The Muslim armies also conquered East Africa and entire Somalia was Islamized. This is one of the greatest invasions  in world history and rivals the campaign of Genghis Khan and Alexander the Great. However there is a subtle difference as both Genghis Khan and Alexander did not do any religious conversion, which the Muslims under the Rashidun and Umayyad Caliphate did with great zeal and  fervor. This was also the period when the Ottoman empire emerged which lasted till the end of World War I.

The Christians were hapless against the invading Muslim force and so were the Hindus. The Muslims brought in the concept of mobility in warfare with hordes of archers on horseback. These circled the enemy and inflicted crushing defeats to their opponents. An excellent description of the military tactics of the early Muslim armies is given by Field Marshal Viscount Lord Montgomery in his "History of Warfare" 

By the 15th century the Muslims had conquered entire India and large areas of Europe. However there was a renaissance in Europe and the Christian armies stopped the Muslim army in the battle of Vienna and slowly began to push them out of Europe. the Moors( Muslims) were also expelled from Spain. As England and France regrouped the Muslims were weakened and in the great Naval battle with  the Ottoman Empire in the 19th century the Europeans were the victors. No such thing happened in India where the Muslim rule saw the Hindu population decrease by 80 million, due to large scale slaughter of Hindus. This was put to a stop with the victory of the East India Company, who defeated the Muslims. The Hindus could now breathe freely.

One of the facts of history that cannot be overlooked is that the early Muslim armies spread Islam by the sword, but once Muslim power was broken they continued their spread by Sufi mystics who converted  Bengal, Indonesia and Mindao to Islam by preaching the message of Muhammad.