Well cock, I lost the original post.
That would have been quite the reasonable attitude to take - in 1550. In 1870 there was precisely no chance in hell that the Spanish and Prussian crowns would be united, even if the Prince of Prussia had succeeded to the Spanish throne instead of the actual cadet-branch prince that was suggested. There was also essentially no reason for the Spanish and Prussian governments to even cooperate unless it was already in their interests to do so. States by 1870 had ceased to act solely for the benefit of the monarch (you could even argue that they hadn't even done so in the past, either, but that's obviously debatable and even irrelevant). Even earlier, having a Bourbon on the Spanish throne had not prevented the Spanish from working with Napoleon. One of the key moments in the death of monarchic geopolitics in Europe was the Baden-Bavaria crisis of the late 1810s, when Bavaria claimed certain territories in the Grand Duchy of Baden upon the death of the Badenese grand duke, and Baden's parliament simply wrote a clause into the constitution of the state forbidding any of its territory to be alienated on succession. Bavaria was roundly mocked by the other states of Europe and eventually had to acquiesce.
The French knew this in 1870. They weren't stupid, although it's easy to portray the French government in 1870 as precisely that. Napoleon and his ministers used Carlos I/Karl V as a nationalistic bogeyman for the media, not as a serious representation of their fears. It was they, and not Otto von Bismarck, who wanted a pretext for war in the summer of 1870, and the Hohenzollern Candidacy provided a cover for what could not be openly stated: the French government's self-perception that it was a Great Power in relative decline in both power (on land to Prussia-North Germany, on the high seas to the United Kingdom) and prestige (after the abject failures in the Luxembourg Crisis and in Mexico). It is a remarkably similar position to that of Austria-Hungary in 1914. (The key difference is that Austria-Hungary also faced serious issues of internal cohesion. France may have as well, but they were clearly not as severe.) So when another opportunity arose - no matter how implausible - to launch what was essentially a face-saving war, the French grabbed it with both hands.
