The negotiations for a marriage alliance between the British Stuarts and Spanish Habsburgs were not a sideshow, a later development, or an ultimate act of desperation by an apparently overmatched king in the early 1620s. Rather, they were absolutely central to the entire connection right from the beginning of James VI and I's English reign and before: from his attempts to secure his eventual accession to the English throne in the 1590s, through the achievement of peace with Spain in 1604, on through the subsequent attempts for the rest of the reign to strengthen the relationship, in order to secure his and his kingdom's position both at home and abroad. A closer look at this relationship underlines just how important the process of making peace and the interconnected early history of the attempts at marriage alliance were for the formation of the entire postwar diplomatic system and for its operation in later years.

Before 1603

When Philip III first heard of Elizabeth I's death and the accession of James I in April 1603, his immediate response was to see if the king of Scots was still interested in a marriage alliance with his family. The overriding foreign policy objective for James had long been the attempt to secure his position as Elizabeth's successor, and the history of a British–Spanish match has its origins in this. He had tried to cultivate support wherever he could find it, at every point across the political and confessional landscape of Europe. To this end, beginning at least as early as 1596 and continuing right on through Elizabeth's final illness, James had had surrogates in Spain and Rome dropping hints and discussing the possibility of marrying at least one of his children off to one of the Catholic king's closest relatives, with the leading candidates being the princes and princesses of the house of Savoy.

In the event of such a match, the Habsburgs would actively support James's English accession with money and troops, in return for a defence of the Catholic cause in Scotland, England and Ireland. James's cross-confessional appeal for support in Europe had led many to believe that his accession might very well be followed by his conversion to the Roman church, and these hopes did not entirely disappear until several years later.