Henry VIII and Catherine of Aragon were Roman Catholic, and the Church forbade divorce. As time went on without a male heir (they were married for 23 years), Henry began looking for a new wife who could bear healthy sons. Young and sophisticated Anne Boleyn caught his eye.
Since divorce was out of the question, Henry petitioned Pope Clement VII in Rome for an annulment — which essentially declares that the marriage never existed. If the marriage never existed, then there could be no divorce, right? Catherine refused to accept this arrangement.
Pope Clement denied an annulment for several reasons, one being that Catherine's nephew, Emperor Charles V of Spain, had laid siege to Rome and essentially was holding the Pope as prisoner.
Henry broke away from the Roman Catholic Church by naming himself the Supreme Head of the Church of England (thereby rejecting the Pope's authority), divorced Catherine, and married Anne Boleyn. Anne failed to produce a male heir and was beheaded after three years of marriage. She was queen for just 1,000 days.