Columbus believed the world was round and that by sailing west from Europe he would eventually end up in India. India was a rich source of trading for Europe. Ships of Columbus' day had to said around the tip of Africa, which was quite dangerous, in order to reach India or return from that country. Queen Isabella and King Ferdinand both realized that if they could find a faster and easier route, they could make a lot of money. Columbus was correct that the earth was round. Unfortunately, he did no realize there were two vast continents between India and Spain when travelling west. Of course we know that North and South America lie between Spain and India if one is travelling west. When Columbus landed on a Caribbean Island, he thought the people he saw were from India. That's why Native Americans were given the name Indians, even though the real India lay thousands of miles further.
Pythagoras knew in 500BC that the earth was round. Columbus was tasked with finding a westward trading route to China and India to avoid the Muslim pirates that routinely robbed ships and cargo, and enslaved or killed the crews of merchant ships traveling around Africa.
The United States began by paying a "tax" to avoid plunder of its ships, but soon tired of appeasing the barbarians. The ensuing battles is where "the shores of Tripoli" comes from in the Marine Corps Hymn, referring to the first Barbary war (as in Barbary pirates). The term "Leathernecks" stems from the thick leather "collars" that were worn by the Marines to fend of Muslims who would "smite at their necks" as they were instructed by Mohammed.