Everyone wonders.
I am sure the Empire was already rotting when Napoleon I lost power.
Anyway, Wellington did wonderfully well in Spain Portugal because HIS communication and supply lines WERE assured by the British navy. No one and nothing interfered with what he needed in the field. He was supplied totally by sea at secure Portuguese and Spanish ports.
Also, Spain (Iberia) was called Napoleon’s ‘ulcer’. He led then SENT a jaded and discordant force into that country and the population took full advantage of everything that kind of confusion and discord does to an army.
The Church was instrumental in organizing the population into destructive mobs and insanely courageous individuals. Even the women waged war. The people were your friends in the daytime and enemies after dark. Some of the worst atrocities ever known in modern fighting were begun in that fight. It was the first truly guerilla fight in recorded history.
Worse still, many of the Marshals sent there DID NOT want to be there, didn’t agree with or comprehend the politics, etc. Many were long-standing rivals and enemies to one another. They never got along and this was such a problem that some even outright sabotaged the plans and efforts of others.
I think it was Marshal Soult who even wanted to set up a separate kingdom for himself, there. Crazy!
The stand out Marshals and the real star performers in the French military, who were unlucky enough to be deployed there, got in and back out of that place with AT LEAST their reputations intact. Not so for most of them ... and in the end it was a disastrous failure.
Reading all that is better than any fake Hollywood drama!
I agree. Knowledgable post.