Just months after Christopher Columbus returned to Europe from his maiden voyage to the New World, the Spanish-born Pope Alexander VI gave Spain a head-start in the quest for domination over newly discovered regions of the world.
The Lands of Spain
The Pope decreed that all lands discovered west of a meridian 100 leagues (one league is 3 miles or 4.8 km) west of the Cape Verde Islands should belong to Spain while new lands discovered east of that line would belong to Portugal. This papal bull also specified that all lands already under the control of a "Christian prince" would remain under that same control.
Negotiating to Move the Line to the West
This limiting line made Portugal angry. King John II (the nephew of Prince Henry the Navigator) negotiated with King Ferdinand and Queen Isabella of Spain to move the line to the west. King John's rationale to Ferdinand and Isabella was that the Pope's line extends all around the globe, thus limiting Spanish influence in Asia.
The New Line
On June 7, 1494, Spain and Portugal met at Tordesillas, Spain and signed a treaty to move the line 270 leagues west, to 370 leagues west of Cape Verde. This new line (located at approximately 46° 37') gave Portugal more claim to South America yet also provided Portugal with automatic control over most of the Indian Ocean.
Treaty of Tordesillas Accurately Determined
While it would be several hundred years before the line of the Treaty of Tordesillas could be accurately determined (due to problems determining longitude), Portugal and Spain kept to their sides of the line quite well. Portugal ended up colonizing places like Brazil in South America and India and Macau in Asia. Brazil's Portuguese-speaking population is a result of the Treaty of Tordesillas.
Portugal and Spain ignored an order from the Pope in enacting their treaty, but all was reconciled when Pope Julius II agreed to the change in 1506.