I was reading about William of Orange on this subreddit today and ended up going down the wikipedia rabbit hole.
I ended up reading about the War of the Pyrenees. The listed reasons for the war are vague and seemingly lacking: "France executed King Louis XVI by guillotine, enraging other monarchs of Europe."
That would be an acceptable reason for Spain to declare war on France, but it doesn't explain why France would suddenly declare war on Spain, who according to wikipedia was France's "ancient ally."
Was France and Spain's alliance really considered ancient? Who was considered their common "enemy"?
Why, when already at war with Habsburg Austria, Prussia, Sardinia-Piedmont and with a pending diplomatic break with Great Britain, did France declare war against Spain?
France and Spain had both been ruled by the same royal family, the Bourbons, since the War of the Spanish Succession; the two nations had been joined by this alliance (formalized in three Pactes de Famille) for more or less the entire eighteenth century.
As to what was going on, note that this war breaks out during the French Revolution -- the execution of King Louis XVI was in essence the state-sanctioned murder of a cousin of Charles IV, during a revolution that posed an existential threat to the traditional monarchies of Europe. How familiar are you with the French Revolution? I don't want to be too pedantic with anything you are already familiar with, but that's the setting for this war. Revolutionary France spent quite some time at war with much of the rest of Europe, which fed more or less directly into Napoleon's rising career and the wars that he himself undertook.