After a long series of wars and battles between the Christian Kingdoms and the Muslim Moors for control of the Iberian Peninsula, King Ferdinand and Queen Isabella finally conquered the Muslims for the last piece of land they owned (Granada). Tariq ibn Ziyad the Islamic ruler backed down and gave in the key for Granada.
This created the Inquisition. The inquisition was both a civil and religious court so if any Jew or Muslim was taken to court and were guilty of not being a "true Roman Catholic" were punished (confinement to dungeons, torture, death)
The Roman Catholic faith was the only true faith in Spain. Queen Isabella and King Ferdinand took this to their advantage. Isabella believed that the country would be united only if everyone followed one religion and the king saw this as an opportunity to control the people.
In 1492 Christopher Columbus sailed west across Atlantic reaching land in the Caribbean claiming it for Spain
The Spanish monarchy funded explorations to find new lands and to establish new trade routs to Asia, the routes around them had been already explored by other countries. They looked in two directions for the expansion of their empire : towards the Mediterranean and west across the Atlantic.