[deleted]
/u/wolfbananabear
I'll cite a previous post.
Charles I and his father James I of England (VI of Scotland) had pursued a Spanish Match strongly from 1614 to 1623, where the idea was for him to marry a Spanish bride -- Infanta Maria Anna of Spain the daughter of King Philip III -- so that the English monarchs could gain significant dowry and thus strengthen their own domestic position against the House of Commons. James I (VI of Scotland) had built their reputation as Protestant kings with good relations with Catholic subjects. Of course, to some extent this was supported by Catholic (recusant) factions within England itself.
Unfortunately, there was still significant opposition within England, which memory of Catherine of Aragon and her daughter Mary Tudor's iron rule was not forgotten -- not to mention more recent warfare including the several Spanish and English Armadas. Protestants called for enforcement of England's anti-Catholic laws and even further direct war against Spain. This was at a time when the Thirty Years War was just starting to brew on the continent, with the defenestration of Prague and conflict in the Palatine. So it was recognized within England itself that such a strong move may stoke the embers of war.
On the Spanish side, the possibility of gaining an English ally is strongly recognized, in particular as Philip III needed to increase his revenues to address domestic weaknesses. In this period, Philip III sought a highly decentralized policy whereby leaders such as Spinola in the Low Countries and Fuentes in Milan were operating as semi-independent leaders of Spanish domains there. If an alliance with England could be realized, the prospective was for Spain to obtain maritime security in the Atlantic and isolation of the Dutch during the Twelve Years' Truce; in exchange for non-interference in England, Scotland and Ireland, and a substantial dowry. At the risk of spoiling the ending, this strategy did achieve some success, as the when the Truce was not renewed, Philip IV could pursue an aggressive strategy that put economic warfare first.
However, among Spanish nobility there was also strong push to demand that Charles convert to Catholicism. So there was a lot of supporters and detractors on both sides. During the Spanish Match negotiations, Spain was not yet directly a belligerent in the Thirty Years War, which was initially seen not necessarily as a religious conflict but rather a political one. Here, Spain had made highly pragmatic overtures in the past, including the offer to pay the English Catholic recusants' taxes that were levied because they didn't attend the Church of England.
Ignoring public opinion, in 1623 Charles decided to travel to Spain to woo the Infanta in person. He strongly felt he needed the Spanish match to gain power and funds as conflict in Germany was brewing. This visit lasted several months, and by all accounts it went very poorly for Charles. Philip III had passed and his successor Philip IV wasn't supportive of the potential match. Charles presented himself poorly to the Spanish court and to the Infanta. Which is too bad because Spain could have rationalized the match based on the threat that France presents to them, and the need to have a friendly England to enable them the pacification of the Dutch rebellion.
Even throughout the Thirty Years War, and following Charles' marriage to a French bride, there were overtures and hopes that Charles might still become an ally, but over time the Spanish court perception became that Charles was a poor ruler and ineffective ally if he were ever to become one.