An very useful expose’ on Islamic motivations, from Raymond Ibrahim:
The problem is that modern historians tend to sideline this religious aspect, and focus instead on national identities. For example, we know that for centuries, a great array of “Eastern” people invaded and sometimes conquered portions of Europe. Modern historians give them a variety of names — including Arabs, Moors, Berbers, Turks, and Tatars; other times they call them Umayyads, Abbasids, Seljuks, and Ottomans. What modern historians fail to do, however, is point out that all these groups relied on the same exact jihadist logic and rhetoric that contemporary terrorist groups such as the Islamic State do today. Whether it was the Arabs (or “Saracens”) who first invaded Christendom in the seventh century, or the Turks and Tatars who terrorized Eastern Europe into the eighteenth century — all of them justified their invasions by citing Islamic teaching, namely, that it is Islam’s “destiny” to rule the whole world through the means of jihad. They also followed the classical juridical injunctions of, for example, offering the “infidels” three choices before battle — conversion to Islam, acceptance of dhimmi status and payment of tribute (jizya), or death. And, once they conquered a Christian area, they immediately destroyed or transformed churches into mosques, and sold whichever Christians were not slaughtered into abject, and often sexual, slavery.
…Jihad against infidels is indeed an integral part of Islam, documented and validated everywhere — in the Koran, hadith (and subsequently Sunna), and the consensus of the umma. No authoritative Muslim cleric (or ‘alim, singular for ‘ulema — “they who know”) past or present, has ever denied this — except, of course, when speaking before “infidel” audiences and practicing taqiyya.