The history of Spain dates back to the Middle Ages. In 1516, Habsburg Spain unified a number of disparate predecessor kingdoms; its modern form of a constitutional monarchy was introduced in 1813, and the current democratic constitution dates to 1978.
After the completion of the Reconquista, the Crown of Castile began to explore across the Atlantic Ocean in 1492, expanding into the New World and marking the beginning of the Golden Age under the Spanish Empire. The kingdoms of Spain were united under Habsburg rule in 1516, that unified the Crown of Castile, the Crown of Aragon and smaller kingdoms under the same rule. Until the 1650s, Habsburg Spain was among the most powerful states in the world.