Thomas F. Madden in his New Concise History of the Crusades:
For a thousand years after the death of the Prophet, the Dar al-Islam, the Islamic world, continued to wage jihad successfully against the Dar al-harb, the abode of war. In that time Muslim armies conquered three-quarters of the Christian world, despite the efforts of generations of crusaders to halt or turn back the relentless advance. An impartial observer at the time might well have concluded that Christendom was a doomed remnant of the ancient Roman Empire, destined to be supplanted by the more youthful and energetic religion and culture of Islam. Yet that observer would have been wrong. Within Europe new ideas were brewing that would have dramatic and unprecedented repercussions not just in the Mediterranean, but across the entire world. Born out of a unique blend of faith, reason, individualism, and entrepreneurialism, those ideas produced a rapid increase in scientific experimentation with immediately practical applications. These included such world-changing devices as the printing press, gunpowder weaponry, and ocean-going vessels. By the seventeenth century European wealth and power was growing exponentially. Europeans were entering a new and unprecedented age.
It is one of the most remarkable events in history that the Latin West, an internally divided region seemingly on the brink of conquest by a powerful empire, suddenly burst forth with amazing new energy, neutralizing its enemies and expanding across the globe. Amazingly, the specter of advancing Muslim armies, which for centuries had posed such danger, no longer constituted a serious threat. Indeed, as the gaze of Europeans spanned new global horizons, they soon forgot that such a threat had existed at all. The Muslim world was no longer viewed as a dread enemy, but simply one more backward culture. From that perspective the medieval crusades seemed distant and unnecessaryâ€”a discarded artifact from the childhood of a civilization.