Jerusalem fell to invading Muslim forces in 638 AD. It was conquered by force of arms, not by gentle persuasion and enlightened preaching (as some modern commentators suggest) after a year long siege. It would be 1099 AD or 461 years before it was returned to Christian hands. That over four hundred year gap between the Muslim conquest and the Christian liberation has led many to argue that 1) Christianity didn't really care all that much about Jerusalem, 2) after so much time it has become a Muslim city, and so 3) the First Crusade was not defensive or liberating but rather offensive and aggressive. It is, therefore, worthwhile to look at that "461 year gap" and see what happened between the Muslim conquest and the Christian re-conquest of Jerusalem.
But first, let us recall just how Christian Jerusalem was. First and foremost, of course, it was the site of Christ's crucifixion and resurrection, and a small Christian population lived in the city from the time of Christ onwards. Admittedly, it remained a predominantly Jewish city, despite the destruction of the Second Temple in 70 AD, until the Romans expelled the entire Jewish population after renewed insurrection in 135.
Jerusalem was then rebuilt by Hadrian, given a new name (Aelia Capitolina) and Roman temples were built on the site of the old Jewish Temple and on the sites sacred to Christians. The objective was to humiliate Jews and Christians alike and, in the case of the Christians, to wipe out the association with Christ's life and certain sites. Furthermore, both Jews and Christians were expelled from Jerusalem and persecuted. Aelia Capitolina was a pagan city, and as such it was nothing more than provincial backwater of little importance to the Roman Empire.
All that changed after the Emperor Constantine came to the Imperial throne. His mother, Helena, was Christian, and she persuaded him to end the persecution of Christians in 313 and to allow her to make a pilgrimage to Palestine. She is credited with locating the sites of Christ's nativity, execution and resurrection. Little more than a decade later, a massive construction project was undertaken to turn Jerusalem into a major Christian capital. In 326 work began on two magnificent basilica: one in Bethlehem over the site of the nativity and the other in Jerusalem over the site of Christ's grave (the Holy Sepulcher).
For nearly 300 years thereafter, Jerusalem was one of the most important cities of the Eastern Roman Empire. Although unable to compete with Constantinople and Alexandria in terms of trade and industry, it was revered for its sacred traditions. Pilgrims flooded to the sacred sites providing a strong economic base that was reflected in construction of churches, monasteries, shops, inns and residences. The inhabitants of this revitalized city were primarily Christian, although Jews were allowed to return as well. The population exceeded 60,000 -- a very substantial population for this period.
In 614 disaster struck. A Persian army surrounded Jerusalem and took it after a 21 day siege. Aided by Jewish allies, the Persians slaughtered an estimated 26,500 Christian inhabitants and enslaved an additional 35,000. The Church of the Holy Sepulcher was raised to the ground. In an ironic twist, the Church of the Nativity in nearby Bethlehem escaped destruction because the mosaic Adoration of the Magi over the portal depicted the Magi as Persian kings; the Persian troops stayed their hand out of respect for the "Persian" kings.
Thirteen years later in 627, Emperor Herakleios wrested control of Jerusalem back from the Persians after defeating them decisively at the Battle of Ninveh. The treaty following the battle required the Persians to withdraw from all conquered territories, including Palestine and so Jerusalem. Yet while Byzantine control over Jerusalem was thus restored, the destruction of the city's sacred monuments and the slaughter or enslavement of the inhabitants could not be so easily overcome. All the Emperor could do was start a rebuilding and resettlement program. In punishment for their role in the slaughter and destruction of the Christian population thirteen years earlier, however, the Jews were again expelled from Jerusalem and prohibited from entering.
Thus at the time of the Muslim conquest, the city was exclusively Christian. Furthermore, the fact that despite the terrible losses and destruction, the city held out for a whole year before surrendering to the armies of Caliph Omar I is a testimony to how vigorously the Christian defenders resisted the Muslim attack. In the end, they were too weak -- as was the entire Eastern Roman Empire.
For the next three hundred years, Islamic oppression continued to expand -- by the sword. It is important to note, Islam itself did NOT spread at the same pace because the conquest of territory by Islamic armies led to slaughter, enslavement and oppression but not necessarily conversion. Indeed, within the next fifteen years alone Syria, Persia, Anatolia, Egypt and Libya fell. These loses crippled the economy of the Eastern Roman Empire, and in 655 the Byzantine navy was also effectively destroyed in a major engagement that left Constantinople incapable of providing support to the far-flung outposts of the Eastern Empire.
The following year, however, the Shia-Sunni split led to the first civil war within the Dar al-Islam (the House of Islam) lasting from 656-661. At roughly the same time, Arab invaders encountered serious resistance from the Berbers in North Africa.
By 678, however, the forces of Islam were again so powerful that they launched an assault on Constantinople itself. The Byzantines fought off the assault with the aid of their massive walls and the use of a new weapon which became known as "Greek fire" - a napalm-based substance that was delivered in pottery vessels that broke on impact resulting in fires that could not be extinguished by water. The attacking Arabs suffered such severe losses that they agreed to a thirty year truce in the wake of defeat. Constantinople was temporarily saved, but the Eastern Roman Empire was in no position to defend its remaining Mediterranean territories, much less undertake an offensive to regain what had been lost. In 698, the mighty (Christian) city of Carthage fell to the advancing Muslim forces and by 700 Islam was ready to turn its violent tactics of "conversion" on Western Europe.
Attacks on Sicily and Sardinia are recorded as early as 704 and Corsica fell in 713. More important, of course, the invasion of the Iberian peninsula began in 711. By 720 the Muslims had forced the Christian defenders into the mountains of the northwest and, dismissing them as a no longer viable fighting force, crossed the Pyrenees to start subjecting the land of the Franks.
In 732, outside of Tours, a Frankish army decisively defeated the invading Muslims in a desperate defensive battle. The Franks furthermore continued fighting the invaders, finally driving them back across the Pyrenees a generation later in 769. By 795 Charlemange had taken his forces over the Pyranees to assist the Spanish Christians in regaining their territories as well. The Reconquista had begun. In short, in the 8th century Western Christians joined Eastern Christians in opposing the brutal invasions conducted against them in the name of Islam.
Meanwhile, Constantinople was still fighting for its very survival. In 717 a new Muslim force by land and sea appeared outside of Constantinople and a year-long siege ensued. After a desperate fight, the capital of the Eastern Roman Empire fought off the besiegers, but it remained mired in a struggle for survival. There could be no thought of freeing something as distant as Jerusalem when Anatolia was constantly raided and plundered. It was not until 740 that the Byzantine victory at Acroinon provided the Eastern Roman Empire with a degree of security in the Anatolian heartland.
The Byzantine victory at Acroinon notably coincided with a general decline in the power and strength of the Umayyad dynasty, which was also beset with problems on its eastern frontiers. This allowed the Eastern Roman Empire to at last start a "reconquista" of its own. In 746, Constantinople regained control of Syria and Armenia, but already by 781 the Byzantines were again on the defensive. For the next half century, the Byzantine Empire was locked in yet another bitter struggle in Anatolia.
Meanwhile Arab rule of the conquered Christian territories from Syria to Spain was characterized by brutality, oppression and humiliation for their majority Christian subjects. (See The Myth of the Andalusian Paradise.) The small Arab elite ruled initially over populations that were overwhelmingly Christian. Due to the burdensome taxes, humiliations and oppression, however, more and more people chose to abandon their faith for the sake of economic gain. Yet conversion is a far slower process than invasion and occupation. To this day, even after 1,400 years of Muslim rule, there are significant Christian minorities in Syria, Lebanon, and Egypt. Ottoman census data shows that in the region around Jerusalem, many villages were still 100% Christian in the early 20th century. Historians estimate that after four hundred years of occupation the inhabitants of formerly Christian territories were still at least half Christian.
The plight of the oppressed Christians population (whether majority or large minority) remained, therefore a motivation for the recovery of lost territory and by the mid-9th century, the Eastern Roman Empire had recovered sufficient strength to launch a sustained "reconquista." In 853 Constantinople sent a fleet to attack Damietta in the Nile Delta. Thereafter, despite some setbacks, the Byzantines continued to regain lost territory right through the middle of the next century. In 943 they liberated Mesopotamia with its overwhelmingly Christian Armenian population. In 961 they recovered Crete and in 965 Cyprus. In 969 Antioch was at last freed from Muslim rule and Aleppo offered tribute to Constantinople to avoid a similar fate.
The recovery of Jerusalem now seemed possible, and Constantinople was determined to regain this most sacred of all Christian cities. A series of campaigns were launched that systematically recovered the coast of the Levant including Beirut, Sidon, Tiberias and Nazareth. Acre and even Caesarea were returned to the Eastern Empire, but Jerusalem remained just out of reach. As the tenth century came to a close, the Byzantines lost momentum and their attempt to regain their lost territories faltered.
What followed was the worst phase yet for subject Christians in Palestine. The new and powerful Shia Fatamid Caliphate pushed back their Sunni rivals and took control of Palestine, including Jerusalem. The Caliph al-Hakim, who ruled from 996-1021, persecuted Christians and Jews and destroyed what was left of the Church of the Holy Sepulcher.
In the West, however, the set-backs had started sooner. In 827 the Muslim conquest of Sicily commenced and although it would take until 902 to complete it would eventually be successful. Meanwhile, in 837 a Muslim army had landed on the Italian mainland, ironically at the request of the Duke of Napes who wanted help in his squabbles with his local enemies. Throughout the rest of the century, the various Italian cities remained divided among themselves and all too ready to accept Muslim assistance, which in turn opened the doorway to Muslim mercenaries sacking, pillaging and pirating from bases in Italy. In 846 Rome itself was attacked by a Muslim raiding force and the basilica of St. Peter was looted but not destroyed.
When three years later a larger Muslim fleet set out to attack Rome again, however, it was met by a combined Christian fleet that defeated it. What followed, however, was not peace but rather a long struggle for control of the Italian mainland. Indeed, the Muslims succeeded in establishing a base for raiding on the coast of Provence at La Garde-Freinet in about 888. While neither the raids from Italy or the base in Provence were comparable to the great Muslim conquests of the 7th century, they posed a menace to travel and trade and kept Western Christendom on the defense.
This did not end until 915 when an alliance of Roman and Byzantine forces drove the last Muslim strongholds off the Italian mainland. For a time, however, the Muslims continued to raid the Italian coastal cities. In 934/35 Genoa was sacked, its male population massacred and the women and children carried off into slavery. Pisa beat off attacks in 1004, 1011, and 1012. Four years later, Salerno came under siege and was only rescued by a band of Normans -- notably on an armed pilgrimage to Jerusalem.
It was only now, at the start of the 11th century, that the tide began to turn in favor of the Christians in the West. The Italian city states were gaining sufficient wealth to finance stronger defenses. In 1034, the Pisans launched an attack on Muslim North Africa. A generation later the Pisans again raided Muslim territory, this time Palermo in 1062 and 1063. Finally, in 1087, a combined force raised from Pisa, Genoa, Rome and Amalfi struck at the main base for many Muslim pirate attacks on Italian ships and cities: Mahdia in what is now Tunisia. The expedition was so successful that it enabled the victors to free prisoners, obtain huge reparations payments, and gain trading privileges. Most important, after the raid on Mahdia, Muslim attacks on Italy ceased almost entirely.
But just as the Western Christians were gaining strength again, the Eastern Roman Empire underwent a new crisis. The Seljuk Turks had converted to Islam and with the passion of the newly converted and the skills of nomadic warriors they set about establishing their domination over Syria and then turned on Armenia, Cilicia, and the Levant, driving the Byzantines out, before striking at Anatolia. The Byzantine Emperor Romanus IV Diogenes assembled his forces and rushed to the defense of this vital heartland -- only to be decisively defeated on August 26, 1071 at the Battle of Manzikert. Shortly afterwards the embattled Byzantines started sending appeals for help to the apparently now stronger West. That aid would, a quarter century later, materialize in the form of what we have come to call the First Crusade.
In summary, Christendom did not "wait" four hundred years to respond to the loss of Jerusalem. On the contrary, throughout the four hundred years between the fall of Jerusalem in 638 and the First Crusade in 1095, Christendom had been fighting perpetually -- and often desperately -- for its very survival. The First Crusade was not "late" response to the fall of Jerusalem, but rather the first viable -- and even so highly risky and audacious -- attempt to retake the city of Christ's passion that had never, for a single day, been forgotten by Christendom.