Almost the whole world that is islamic became muslim through military occupation, the mass slaughter of the males who fought or would not convert, the looting of property and the selling off into slavery of their women and children. This was all based on emulating the behaviour of Mohammed (known as sunnah in islamic practice); Mohammed took 20% of the spoils from his army of warriors. (Some describe him as the profit of 20% because of this). This can be verified in The Life of Mohammed, the first biography of Mohammed (and written by a muslim 100 years after the death of Mohammed). The author, Ibn Ishaq, was a christian who was taken as a slave to Medina, and converted to islam for a better life (one of the inducements used in the spread of islam).
The real lesson is that christianity endured 300 years of this aggression, before Pope Urban made christianity do an about turn, and began the (very minimal) Crusades against islam. They were a failure, and islamic violence against European christendom continued until the Siege of Vienna (when Jan Sobieski defeated the muslim army on September 11, 1683).
Watch just 1 minute of this video to see what happened:
http://www.youtube.com/watch?feature=player_detailpage&v...
The muslim attack against Europe continued beyond this. The white slave trade continued for hundreds of years (it was stopped by a joint British/US naval bombardment of Algeria in 1815). In 1951 the UN report on slavery calculated that 5% of the population of Saudi Arabia and Yemen were still slaves.
Buddhism did not respond in the same way as christianity to islamic aggression. Many experts on the history of buddhism say that buddhism died about 1000 years ago (e.g. Conze). And they explicitly attribute this death of buddhism to the onslaught of the islamic invasion & destruction in India.
The real lesson is that some religions are actually militant political movements wrapped up in religion. Islam is one such religion. Islamic militancy was going on for hundreds of years before christianity responded in kind.