Was there any realistic or worthwhile French goal in the peninsular war?

[Kyriakos]

The peninsular war started at 1808, following Napoleon's decision to install his brother as the new king of Spain. It was preceded by the invasion of Portugal (by France and Spain) and more crucially by a local revolt in Madrid, which quickly led to clashes between Marshal Murat's forces sent to pacify the population and civilians (the first clash, as well as the reprisal, are immortalized in two paintings by Goya; OP shows part of the latter). After Napoleon installed his brother as king, mass revolts (and ultimately a British army arriving) were only a matter of time.

I wish to ask what Napoleon's reason for taking over Spain was. Surely he already had larger and better territories as vassals, and Spain would require hundreds of thousands of French soldiers to guard (as it did historically in that long campaign).
Unless there was a real risk of Spain siding with Britain, I don't see the strategy in what happened. Besides, Napoleon already had narrowly avoided disaster in battle before all this - at Eylau - so he couldn't just have been feeling invincible.

Maybe @Gedemon (as the reason may have been presented in French schools )

 

[Birdjaguar]

Remember hindsight is 20/20.

By December of 1807 Napoleon was in control of Lisbon and most of Portugal but his plans to capture the Portuguese fleet had failed. At that point Napoleon was at the height of his power and needed to enforce the Continental System against Britain. Spanish ports were still open to British trade and Spain was weak and ripe for the taking. He was an opportunist and the Spanish government was faced with both corruption and internal strife between the king and his son. His army was already in Spain and set up in most major cities and the Spanish army considered as a weak opponent. Napoleon created an excuse to pull the trigger on his plan to take over Spain in Feb. 1808. He had two choices: end the Bourbon House of Spain altogether or set up or set up Charles' son, the popular Ferdinand, as a puppet. He chose the former and it was a mistake.

Napoleon's goals were tied closely to enforcing the Continental System against England. He failed to understand the Spanish people and their resistance to a regime change. On April 1, Madrid revolted and it was all downhill from there.

 

[Josu]

In aditon to what Birdjaguar says he wanted to neutralize the Bourbons so that their legitimacy would not extend to the Bonaparte dynasty in France.

 

[Kyriakos]

neutralize the Bourbons so that their legitimacy would not extend to the Bonaparte dynasty in France.

Interesting. Was that fending off a very long-term erosive effect? (because in 1808, Napoleon could field half a million french soldiers so no attack would threaten him anyway- let alone how many germans/polish/others he could enlist).
As for the continental system, besides that it wasn't working anyway, Spain was close enough (and weak enough) that Napoleon could threaten its own dynasty to at least pretend to enforce it.

Of course a problem with Napoleon is that he was in many ways not at all a perfect general (though far better than his concurrent adversaries) - much like his idol, ceasar, who also lost a lot of battles. As early as the italian campaign he could have been ruined (eg at lake Garda where all sorts of luck aided him) and after he became emperor too; there not entirely ruined but still have to contend with a stalemate (late stage of the second Prussian campaign, before Friedland, or early stage of the third Austrian campaign, at Aspern-Essling).

 

[Josu]

Interesting. Was that fending off a very long-term erosive effect? (because in 1808, Napoleon could field half a million french soldiers so no attack would threaten him anyway- let alone how many germans/polish/others he could enlist).
As for the continental system, besides that it wasn't working anyway, Spain was close enough (and weak enough) that Napoleon could threaten its own dynasty to at least pretend to enforce it.

Last French kings to date where Bourbons, same dinasty as in Spain, he was affraid of Spanish Bourbons claiming for legitimacy of French throne.
Carlos IV and Fernando VII were not particularly intelligent, to say the least. Moreover, they were corrupt and sought to cling to power at all costs, which led them to align themselves with Napoleon, thinking it would benefit them. However, Napoleon took advantage of the situation to install his brother as ruler and thus attempt to remove the Bourbons from the equation. Nonetheless, Napoleon underestimated the peculiarities of the Spanish people, who initiated the War of Independence.