Question

what was the reason why Pope Alexander divided world between Spain and Portugal?

Answer 1

The pope[who was Spanish] wanted to give Spain and to a lesser extent Portugal an advantage on exploring the new world.

Answer 2

Also to keep them from arguing with each other other about claim to new lands.

Answer 3

Spain laid the first claim to the New World, but failed to act on that claim immediately, as they were dealing with minor disputes with other European countries. Portugal, meanwhile, was getting a head start on the exploration of the New World. When the Spaniards noticed that their neighbors the Portuguese had begun to colonize the land they had claimed, war seemed imminent. To prevent a conflict between its two most powerful member-countries, the Catholic church, aka Pope Alexander, settled the dispute by dividing the world into two explorable pieces, one for Spain and one for Portugal.