Why did Thomas Cranmer declare Henry VIII’s marriage to Catherine of Aragon invalid?

Thomas Cranmer declared Henry VIII’s marriage to Catherine of Aragon invalid because he was acting under the king's authority to resolve the "divorce" controversy that had stalled with the papacy.123

Henry VIII sought an annulment on the grounds that his marriage to Catherine violated the biblical prohibition against a man marrying his brother’s widow, as Catherine had previously been married to Henry’s brother, Arthur. While Pope Clement VII refused to grant the annulment due to political pressure from Catherine’s nephew, the Holy Roman Emperor Charles V, Henry moved to secure his own ecclesiastical judgment.1

After being appointed archbishop of Canterbury in 1533, Cranmer convened a court at Dunstable. In May of that year, he officially pronounced the marriage to Catherine void from the beginning, thereby facilitating the king's desire to legitimize his union with Anne Boleyn. This act was a pivotal step in the English Reformation, leading to the Act of Supremacy and the formal break with papal jurisdiction.12