I need information on the foreign relations of the Iberian Union within Europe (i.e. their allies, enemies, friends, trading partners, etc.). I can't find ANYTHING about this online, besides the people who hated them.
A reply to /u/Cthulhus_Curios
I think online, meaning only public websites, is probably not the best place to look, although the history of the Iberian Union is quite long and complex. Can I ask if you specifically ask about the period when Portugal was in personal union with the crowns of Spain? If you are, then you may want to look up the history of Philip II, Philip III, and Philip IV. Those were the three monarchs under whom there was the Iberian Union.
More specifically, you can search for the history of the Eighty Years' War, which was a major driving force of politics in Europe in that period. If I have to choose just one article to read, it will be:
For a larger picture, you can search for articles on the Thirty Years' War. The corresponding Wikipedia pages for the 80YW and 30YW do not suck, so they are a decent starting point.
So TL;DR I think you've been searching for the wrong keywords, because in the English language there is probably almost no writing specifically on the Iberian Union, rather people write about the 30YW and 80YW, which are highly relevant in the context of the history of the Iberian Union.