Hear me out: the Peninsula War was a bigger blunder than Russia.

By this, I don't mean the raw damages it caused to Napoleon's Army and to his Empire were bigger. They obviously weren't. I mean that it caused a ripple effect which drastically weakened French position in Europe in the latter years of Napoleon's First French Empire.

Let me explain my position.

I won't bother to explain the context when Napoleon got entangled in the Iberian Peninsula, I think everyone here knows what happened in 1805-06-07 and the consequences of these events.

I will only focus on the consequences of the Peninsula War.
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First, this pointless war weakened Napoleon's position on the continent.

After Trafalgar, it became clear Napoleonic France couldn't challenge the British on the seas, so Napoleon had to focus on the continent. His position after Tilsit was incredibly strong. His Empire was immense, stretching from the Atlantic Ocean to Poland.

The Peninsula War drained Napoleon's resources and some of his best troops and best commanders(guys like Soult and Suchet). For 6 long years, this war saw Napoleonic France lose more and more experienced troops, troops they couldn't replace.

But this war also had another major consequence:

I would argue that had it not occurred, Austria would not have tried their hand in 1809 against Napoleon. In other words, Austria took advantage of the weakening of Napoleon's position in Europe to launch the War of the Fifth Coalition. I'm sure Austria would've fought Napoleon again, but later. Which leads me to my second point...
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Second, the War of the Fifth Coalition had tremendous impacts on the continent.

Yes, admittedly, Napoleon won it quickly and decisively. That is true. But, his rapid triumph was also marked by major losses. The battles of Aspern-Essling and Wagram saw tens of thousands of French casualties. These were, in large parts, the veterans of the Grande Armée. A lot of these guys had fought at Ulm, Austerlitz, Jena-Auerstedt and Friedland, the 4 greatest feats the Grande Armée accomplished during the Napoleonic Wars. These veterans, like those lost in the Iberian Peninsula, couldn't be replaced.

But what's more, the Fifth Coalition saw Napoleon lose three commanders who were so important to his army:
-Marshal Lannes
-General St-Hilaire
-General Lasalle

Lannes had play key roles at Lodi, Arcole, Montebello, Marengo, Austerlitz, Jena and Friedland.
St-Hilaire's division had in a large part conquered the Pratzen Heights at Austerlitz
Lasalle had led the decisive cavalry charges at the Battle of Rivoli.

Lannes was Napoleon's very best friend, St-Hilaire was perhaps his best divisional commander and Lasalle was another friend of Napoleon. These experienced, battle tested and extremely competent commanders couldn't be replaced. I'd argue both St-Hilaire and Lasalle would've become Marshals had they lived. But they did not.
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Third, imagine for a second a world without the Peninsula War.

Let's imagine the German Campaign of 1813, which Napoleon nearly won anyway, but without the Peninsula War. Imagine both Suchet and Soult commanding in central Europe. Heck, Soult had been in Germany for the opening engagements of the German Campaign, namely Lutzen and Bautzen, but after the debacle at Vittoria, Napoleon was forced to send him back to Iberia.

Imagine Soult commanding in Oudinot or Ney's place at Grossbeeren/Dennewitz. Imagine Lannes commanding at Macdonald's place. Lannes wouldn't have made the foolish mistake of placing his back to a river like Macdonald did. Imagine a world in which Generals St-Hilaire and Lasalle lived and became Marshals, and commanded in Germany in 1813! Plus with the tens of thousands of extra troops Napoleon would've had, he would've won in 1813.

Heck, as I said previously, he almost did anyway. Bautzen came very close to being an overwhelming French victory, Murat's charge at Leipzig almost captured the three Coalition monarchs, and that was without Soult, Suchet, Lannes, St-Hilaire and Lasalle, not to forget the tens of thousands of men that would've fought in Germany in 1813.
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Of course, I am not saying the Peninsula War was worse than Russia in terms of its numbers. Of course for that Russia was way worse, but it was also very quick, where had Iberia was a slow, painful, long drain on Napoleon's forces.

Feel free to disagree, but I thought I'd share some of my thoughts of the Peninsula War and its impact on the Napoleonic Wars as a whole.