The Crusades were a series of religious as well as military campaigns waged by Christians against their external and internal opponents. They were fought mainly against the muslims tough campaigns were also launched against jews, Russian & Mongols. They had a clear goal of capturing Jerusalem and the Holy land from Muslim rule.
Crusades are a series of war between Muslims and Christians ,which were fought to gain the Holy land of Jerusalem and near-by region.When the Turks gain control of Jerusalem and made clean sweep till Asia Minor ,they also captured Constantinople .They made efforts to gain the control of Jerusalem,which was also regarded sacred by the Muslims.At last ,they took Jerusalem into possession.This triggered hostilities in Christian Community and Pope UrbanII summoned all Christians to wage war against the Muslims.In this way ,a series of Holy war started which continued till 12th Century.
By the 7th century, Christianity was widely established in the eastern Mediterranean, from Greece, through Turkey, Syria, Lebanon, Palestine (now Israel) and Egypt.It extended inland eastwards.It coexisted with other religions pretty easily, the principal one being Judaism and the Roman religion of Mithraism.There's plenty about this on the Internet.You'll have to look it up. When Mohammed founded the Moslem religion, it established itself rapidly as hostile to other religions and expanded westwards.Basically, people were given a choice -- convert or die.The entire middle eastern area was overrun and Christianity and Judaism suppressed.The Crusades was a conglomerate army of the Holy Roman Empire that went out to recover the "holy lands" for Christianity.It was an extremely bloody conflict between Christianity trying to carry on and Islam, which was determined to spread westwards, taking no prisoners.
The crusades started in 1096 and ended in 1099 only as the FIRST crusade. There were Eight crusades. They were against The muslims in an attempt for the christians to gain back Jeruselam. Hope that helped.