During the reign of King James I of England, there was a royal marriage proposal that was brought up between his son, the future King Charles I, and the Spanish Infanta Maria Anna, who would later go on to marry Ferdinand III of the Holy Roman Empire. This was intended to be accompanied by a treaty where the Spanish agreed not to interfere in Ireland and England agreed not to allow privateers to screw around in the Caribbean. Additionally, Spain would pay 600k pounds (with inflation that must be a fuckload today) to James so he wouldn't have to make concessions to Parliament for money like what happened with his son years later. The Protestant-dominated Parliament hated the Match because of the recent outbreak of the Thirty Years' War, which, alongside the King's sidelining of native English men for the Spanish would probably cause some great derision between the King and Parliament. Anyway, when the then-Prince Charles and the Duke of Buckingham secretly went to Madrid to arrange the marriage, they found that Maria Anna would only agree if Charles would convert to Catholicism, which he refused to do, as well as repeal the Penal Laws against Catholicism in Ireland. The two then returned to England empty-handed, and Charles would become King in 1625, eventually marrying Henriette Marie of France.
What if Charles and Maria Anna had been married? James would die soon after and he and Maria Anna would be the first Catholics to rule in England in 65 years since the death of Queen Mary, which would piss off the hell out of Parliament and possibly even cause rebellions like those of the Monmouth's when James II ascended the throne. I would assume he would be inspired by the Spanish and attempt to rule as even more of an absolute monarch than OTL, and any rebellions that would sprung up could probably be put down somewhat easily given the Infanta's dowry. I would expect him to pass a law that would permit Catholicism to be freely practiced within England, albeit stopping short of making it the official religion of his kingdom, and when the Bishop's War with Scotland would come up he would probably be more successful in his venture, given that he would have a huge sum of money at his disposal. I could see him sending support to Spinola in the Netherlands against the Dutch, as the English begin to notice the Dutch naval prowess and, given their already-existing alliance with the Spanish, decide to give the Spanish a bit of free help in their war.
Thoughts?