Answer:
The falling of Constantinople by the Ottoman Empire.
Explanation:
Before that, Europeans had enjoyed a safe land passage through the Silk Road. Especially because of the Mongols Empire´s hegemony over Asia. When it fell in the hand of the Ottoman the land route became very dangerous.
Because of that Portuguese sailors tried to find a route across the sea. They first proposed to go around Africa but they came up with the idea than traveling west across the "Ocean Sea", what today is the Atlantic, would be a faster route than the southeast passage.
Columbus proposed a route to reach the East Indies by going west and finally, the Catholic monarchs agreed to sponsor him.
I hope this answer helps you.
