Catherine of Aragon was one of four offspring of
Ferdinand and Isabella, the monarchs of
the Spains.
Catherine had been married to Arthur, son of Henry VII1. Henry VII had been very keen on this marriage, and it represented the biggest success of his foreign policy. Why was Catherine so valuable? Because of her parents.
The Spanish Catholic Kings were two of the most prstiguous monarches in Europe at the time. Henry VII was a young upstart of the as-yet unimportant House of Tudor, who had a limited heredity right to the throne on which he sat and had seized it through battle. To be recognized as an equal by the Spanish made Henry feel a lot more secure on his throne, and provided him a legitimacy which he hoped would make him more than just another temporary monarch, soon to be ousted as part of the War of the Roses.
On 14 November, 1501, Prince Arthur and Catherine were married in St. Paul's Cathedral. Sadly, Arthur died five months after the wedding in Ludlow2, and Henry's dynasty appeared to be in tatters. Yet Ferdinand and Isabella almost immediately began pursuing the marriage of Catherine to the young Henry VIII. This was achieved with a dispensation from the Pope.
The reason this dispensation was needed is important, and a matter of Scripture. Leviticus 20:21 states (King James Bible):
"And if a man shall take his brother's wife, it is an unclean thing: he hath uncovered his brother's nakedness; they shall be childless."
Indeed, Catherine and Henry VIII would never produce a male heir. When this had transpired and she had passed child-bearing age, Henry wanted rid of her to marry Anne Boleyn. Divorce was out of the question3, and so Henry sought to annul the marriage. Thomas Wolsey, papal legate and Chief Minister of Henry, set to work trying to persuade the Pope.
The Pope (Pope Clement VII) was unwilling to accept that one of his recent predecessors had made a mistake in granting the initial dispensation (trying to prove this was the main line of attack made in favour of the divorce). He was also severely chastened after the Sack of Rome and keen to act in favour of Charles V, who of course was the nephew of Catherine. Eventually Henry was only able to get a favourable decision by devolving power through the English Reformation.
For her part, Catherine is commended for her activities and attitudes throughout the divorce proceedings. She never had any doubt about the validity of the marriage and was in constant contact with the Pope and Charles V to try and get a decision in her favour. This active resistance came as a great surprise to Henry VIII.
Notes:
1. Arthur was so named as a propaganda exercise by Henry, keen to use Arthurian Mythology to make himself look better following his seizure of the throne at the Battle of Bosworth.
2. On the issue of consumation, it is generally accepted that the marriage was unconsumated, although Prince Arthur once reportedly boasted that "It's hot work spending the night in Spain" to his courtiers.
3. Despite his behaviour, Henry VIII was by all accounts a pious Catholic, and it is accepted that he did in fact fear that he and Catherine were living in sin.