Centuries before the Crusades (they began around 1095 A.D.) and even decades before the Islamic invasion of Spain (that was in 711 A.D.) the newly-minted Muslim armies were on the march.
Muhammad died in 632 A.D., and even before his death, Islamic invasions were conquering large parts of the ANE (Ancient Near East). After his death, the Muslim military continued to gain new territory.
Due to their superior horsemanship and swordsmanship, the Muslims managed to expand their empire - their ‘caliphate’ - rapidly. They moved westward, conquering large parts of northern Africa. They advanced toward the northeast, defeating Syria and moving into Persia.
Islam next set its eyes on Europe. Historian Rodney Stark writes:
Having defeated the Byzantine armies in Syria and Egypt, and having begun a successful campaign to conquer the entire north coast of Africa from Byzantium, in 672 the caliph Muawiyah decided to strike directly at his enemy. From his new capital in Damascus, the caliph directed his fleet to transport an army through the Dardanelles (the narrow strait linking the Mediterranean Sea with the Sea of Marmara). Numbering about fifty thousand men, the caliph’s troops captured the peninsula of Cyzicus, across the water from Constantinople, and fortified it as their principle base, from where they began a siege of Constantinople.
This siege, from the principal base across the water, was unsuccessful.
After this attack in 672 A.D., the Muslims tried again in 717. Defeated but persevering, they assaulted the city of Constantinople again in 718. In fact, Islam assailed the city repeatedly for several centuries. It finally fell in 1453.
The assaults on Constantinople were part of a three-front strategy. Islam hoped to stretch Europe’s defenses thin by striking from the southwest at Spain, from the south at Italy and Sicily, and from the southeast at Constantinople.
This left Europe with thousands of miles of coastline to defend: a difficult task at best, if not simply impossible.
After enduring decades of attack all along the Mediterranean south, and offering ad hoc defenses, Europe decided to organize a strategic counterattack: to stop the invasions at their source in the Middle East, rather than wait for Islamic armies to arrive yet once again on European shores.
This defensive counterattack would be called ‘The Crusades,’ but that title would not be given to it until centuries after it happened in 1095 A.D.