The major political event that led to the decline of Spain's influence in Europe was the Spanish-American War in 1898. This conflict resulted in a decisive victory for the United States and marked the beginning of American imperialism. Spain lost its colonial possessions in the Caribbean and the Pacific, including Puerto Rico, Guam, and the Philippines. The war not only diminished Spain’s status as a world power but also catalyzed movements for independence in its remaining colonies. This shift significantly altered the balance of power in the Americas and paved the way for the United States to emerge as a major global player.