The first thought I have on reading your post is that things as they were perceived at the time, were not necessarily as they really were.

Second, and a minor point, perhaps, but Wellington's army was far from being entirely British, and there were other armies involved in the campaign from time to time as well as the guerrillas.

I also take your point about central and eastern Europe being the place where Napoleon made his main effort.

However, I struggle to find any decisive campaign in eastern or central Europe between 1805 and 1813. All they did was provide a temporary military success and an interlude. Other than that they decided nothing. The Austrians, Russians and Prussians came back time after time until the decisive campaign of 1814 in France which resulted in the fall of France and, the escape from Elba notwithstanding, what we would call 'regime change' today.

The argument, as I understand it, is that had the Peninsular been resolved before 1813 the number of troops available for the 1813 campaign in Germany would have been sufficiently greater to make a difference to the outcome.

One could expand the argument to something like, had there been no campaign in Spain at all, Russia would not have been inclined to defy France and the 1812 campaign would not have been necessary and therefore, the campaign of 1813 would not have taken place, and neither would the decisive campaign of 1814.

Be that as it may, and keeping it simple, had Spain been resolved by 1813, the number of troops available might have been sufficiently greater to defeat the allies in Germany. After all, Napoleon nearly defeated them with what he had available.

If this argument is accepted, and acceptance of an argument is not mandatory (though reading some of the posts elsewhere lately one could be forgiven for thinking that it is), it follows that the Peninsula, because it tied down so many French troops, was fundamentally important to the outcome in the main theatre of war where the outcome might otherwise have been quite different.

So, perhaps we should not look at things in isolation, rather as essential parts of a whole.

JC