Catherine of Aragon served as queen consort of England from 1509 until Henry VIII had their marriage annulled in 1533. Her position joined the English monarchy to the Spanish dynasty of Ferdinand and Isabella. Previously married to Henry's elder brother Arthur, who died in 1502, she spent several uncertain years in England before marrying the new king. Her diplomatic experience supported an active role in government.

Henry appointed Catherine regent while he campaigned in France in 1513. When James IV invaded from Scotland, she helped organize England's defense, mobilizing supplies and communicating with commanders. The English victory at Flodden was won by the army under the Earl of Surrey, rather than by Catherine in person. Nevertheless, her exercise of authority during the crisis demonstrated that a queen could undertake substantial administrative and military responsibilities.

Catherine promoted religious charity and learning, and directed her daughter Mary's education. Her pregnancies produced no surviving son, increasingly troubling Henry's plans for the succession. From 1527 his effort to end their marriage placed Catherine at the center of an international dispute involving the papacy and her nephew, Emperor Charles V. She consistently defended the marriage's validity and Mary's legitimacy. At the legatine hearing in 1529, she appealed directly to Henry and challenged the court's authority.

Henry's break with papal jurisdiction enabled Archbishop Thomas Cranmer to declare the marriage invalid in 1533. Catherine refused to recognize the ruling or accept the reduced status of dowager princess of Wales. Separated from Mary and moved between residences, she continued to claim her position as Henry's lawful wife until her death at Kimbolton in January 1536. Her resistance shaped the constitutional and religious crisis surrounding the English Reformation, while Mary's later accession gave renewed political importance to her defense of the succession.