 The thought that Spain could bring Christianity to the New World and protect Catholicism in Europe certainly played a strong role in the expansion of Spain's empire.Spain's world empire reached its greatest territorial extent in the late 18th century but it was under the Habsburg dynasty in the 16th and 17th centuries it reached the peak of its power and declined.The Iberian Union with Portugal meant that the monarch of Castile was also the monarch of Portugal, but they were ruled as separate entities both on the peninsula and in Spanish America and Brazil.