Islam spread very rapidly across the Arab world in the century after Muhammed's death, with Muslim leaders conquering vast territories and converting them to Islam, a religion whose monotheism was often easier for people to grasp than Christianity's trinitarianism. The Ottoman Empire that in later centuries fanned out from Turkey was part of this consolidation and expansion of Muslim power. Eventually, civil wars within the Byzantine Empire and the continued growth of the Ottoman Empire led to the defeat of Christian Constantinople in 1453. At this point, the Muslim Ottomans controlled both land and sea access to the Far East.
The overland and Mediterranean sea routes the Europeans had been accustomed to travel to get spices, cloth, and other materials from Asian nations such as India and China were suddenly far more expensive in terms of tolls and tariffs, and far more difficult to navigate due to Barbary pirates on the Mediterranean, than they had been in the past. It would be hard to overestimate the impact of this regime change in Constantinople on world history.
Although the profit had largely gone out of European trade with Far East, the hunger for goods from that part of the world, which the Europeans could not produce themselves, remained very high. Desperate for a solution, the Western European nations, most notably Portugal and Spain, began exploring to find new sea routes to India. Portugal showed this was possible when they navigated around the tip of south Africa and across the Indian Ocean to India.
But the most important explorations were those, such as by Christopher Columbus, that headed west looking for a route to the Indies. At this point, an unexpected event occurred when explorers bumped into the North and South American continents. Access to the wealth and resources of these vast land areas changed the fortunes of Western Europe in incalculable ways, allowing it to amass enough power to dominate the world.