Since the 1st century BC, the border between the Roman (later Byzantine) and Parthian (later Sassanid) empires had been the Euphrates river. The border was constantly contested. Most battles, and thus most fortifications, were concentrated in the hilly regions of the north, as the vast Arabian or Syrian Desert separated the rival empires in the south. The only dangers expected from the south were occasional raids by nomadic Arab tribesmen. Both empires therefore allied themselves with small, semi-independent Arab principalities.

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After the death of Muhammad, founder of the Islamic religion, Abu Bakr succeeded him. But after his death, there had to be another person to continue the spread of Islam. That person was Umar. He set up armies to spread Islam to Syria and Palestine and ended up winning Jerusalem from the Byzantines in 637 AD. That was the year when Jerusalem became Islamic territory. It’s sort of ridiculous to start a war over religion. It seems like an oxymoron, because Islam has a lot to do with respecting God. Killing people to spread a religion doesn’t seem like something God would appreciate.
http://www.truthnet.org/islam/whatisislam.html
http://www.bookrags.com/wiki/Islamic_conquest_of_Persia