By the late 15th century, European marine technology had advanced to the point that long ocean-going voyages were possible. Ships stout enough to withstand the perils of the Atlantic had been designed, new arrangements of lateen and square sails meant harnessing the wind more effectively, and navigational skills and aids such as the compass and astrolabe had permitted sailing into waters far from familiar coastlines.
This enabled nation states on the Atlantic coast to invest in exploration whose purpose was to find a sea-route to Asia and its trade riches. The country that achieved this might thus cut out Mediterranean middlemen like Venice and Genoa and avoid dealing with hostile Islamic powers.
Portugal was first to take up this challenge, and undertook a series of expeditions down the dangerous and unexplored coast of Africa, compiling maps and charts of the winds, and establishing bases. In 1488, ships led by Bartolomeu Dias finally passed the Cape of Good Hope, reaching the southern tip of the continent. Within a decade, Vasco da Gama would go farther, sailing north to reach India, upsetting centuries-old trading patterns.
The Genoese navigator Christopher Columbus had attempted to interest the Portuguese in a scheme to reach the Indies, not via Africa but by a westerly route. Scientific advisers in Lisbon, however, realized that Columbus had seriously erred in his estimation of the length of such a journey and sent him packing. He found a more sympathetic hearing from Isabella, the Queen of Castile, the leading Spanish power. Together, Isabella and Columbus negotiated a deal that would provide him with a small fleet, a cut of the profits from his voyage, and the grand title of Admiral of the Ocean Sea.
In August 1492, Columbus set sail toward the west and eventually blundered into hitherto-unknown islands, convinced that he had reached the outskirts of Japan. This serendipitous miscalculation proved to be the discovery of the Americas, from which the Spanish would reap golden dividends.

“Among other works well pleasing to the Divine Majesty and cherished of our heart, this assuredly ranks highest, that in our times especially the Catholic faith and the Christian religion be exalted and be everywhere increased and spread, that the health of souls be cared for and that barbarous nations be overthrown and brought to the faith itself,” the papal bull stated.
“We … assign to you and your heirs and successors, kings of Castile and Leon, … all islands and mainlands found and to be found, discovered and to be discovered towards the west and south, by drawing and establishing a line from the Arctic pole, namely the north, to the Antarctic pole, namely the south, … the said line to be distant one hundred leagues towards the west and south from any of the islands commonly known as the Azores and Cape Verde.”
The Portuguese were unhappy with this rather vague division of the globe and saw that it not only precluded their hopes of eventually claiming rights in India but also violated earlier treaties that had given them all lands south of the Canary Islands. King John II warned the Spanish that he was prepared to go to war over this issue and that he was preparing to send an armada to seize whatever lands Columbus had found.
The Spanish were willing to negotiate and ignore, or at least amend, Alexander VI’s papal bull; the result was the 1494 Treaty of Tordesillas. This agreement followed the notion of a north-south line down the Atlantic Ocean but moved the demarcation to the west, giving Portugal territory to the east and Spain the lands to the west (and incidentally nipping off the bulge on the east coast of South America which the Portuguese would colonize as Brazil). More papal decrees and treaties would be necessary before an agreement in 1529 solved most of the Spanish-Portuguese bickering.
With these agreements, the age of globalism and European imperialism had begun. Other European nations tended to ignore these rulings altogether—England, France, and the Netherlands would succeed in muscling in on the territories allocated to Spain and Portugal—and, of course, the native states of the Americas, Africa, and Asia were given no say in the matter.