Excerpts from the book, The Politically Incorrect Guide to Isalm and the Crusades, by Robert Spencer, Regnery Publishing, Inc., Washington, D.C., 2005. (Everything below is directly from the book, with no commentary from me [the author of this web site].)
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Like Damascus and Antioch, Jerusalem was a Christian city at that time. It was the unhappy task of Sophronius, the patriarch of Jersualem, to hand over the city to the conquering Umar. The caliph stood happily on the site of Soloman's Temple, from which he may have believed that the Prophet Muhammad, his old master, once ascended into Paradise {cf. Quran 17:1, a verse that has inspired centuries of debate as to its precise meaning}. Sophranius, watching in deep sorrow nearby, recalled a Bible verse: "Behold the abomination of desolation, spoken of by Daniel the prophet." (page 108-109)

According to conventional wisdom, Byzantine rule was so oppressive on the Christians in the Middle East and North Africa, and Egyptians in particular, that they couldn't wait to give them the bum's rush and open their arms to the Muslims who liberated them from this oppression. But, in fact, the Muslims conquered and held Egypt only in the face of great resistance. In December 639, the general Amr began the invasion of Egypt; in November 642, Alexandria fell and virtually all of Egypt was in Muslim hands. But this swift conquest was not uncontested, and the Muslims met resistance with brutality. In one Egyptian town they set a pattern of behavior that they followed all over the country. According to a contempory observer:

Then the Muslims arrived in Nikiou. There was not one single soldier to resist them. They seized the town and slaughtered everyone they met in the street and in the churches-- men, women and children, sparing nobody. Then they went to other places, pillaged and killed all the inhabitants they found .... But let us now say no more, for it is impossible to describe the horrors the Muslims committed when they occupied the island of Nikiou.

Not only were many native Christians killed-- others were enslaved:

Amr oppressed Egypt.... He took considerable booty from this country and a large number of prisoners.... The Muslims returned to their country with booty and captives. The patriarch Cyrus felt deep grief at the calamities in Egypt, because Amr, who was of barbarian origin, showed no mercy in his treatment of the Egyptians and did not fulfill the covenants which had been agreed with him."

Christian Armenia also fell to the Muslims amid similar butcheries: "The enemy's army rushed in and butchered the inhabitants of the town by the sword.... After a few days' rest, the Ismaelites went back whence they had come, dragging after them a host of captives, numbering thirty-five thousand."

The same pattern prevailed when the Muslims reached Cilicia and Caesarea of Cappadocia in 650. According to a Medieval account:

They [the Taiyaye, or Muslim Arabs] moved into Cilicia and took prisoners ... and when Muawiya arrived he ordered all the inhabitants to be put to the sword; he placed guards so that no one escaped. After gathering up all the wealth of the town, they set to torturing the leaders to make them show things [treasures] that had been hidden. The Taiyaye led everyone into slavery-- men and women, boys and girls--and they committed much debauchery in that unfortunate town; they wickedly committed immoralities inside churches. (page 110) ...

What was the ultimate goal of this seeminly endless warfare? It is clear from the commands of the Quran and the Prophet, who told his followers that Allah had commanded him, "to fight against the people until they testify that none has the right to be worshipped but Allah and that Muhammad is the Messenger of Allah." No Islamic sect has ever renounced the proposition that Islamic law must reign supreme over the entire world, and that Muslims must, under certain circumstances, take up arms to this end. They stopped waging large-scale jihads after 1683 not because they had reformed or rejected the doctrines that motivated them, but because the Islamic world had grown too weak to continue-- a situation that began to change in recent times with the discovery of oil in the Middle East. (page 114-115)