If you're referring to Napoleon's Spanish Iberian peninsula campaign between France and the coalition of Spain, Portugal and England from 1808-1814 there were three critical reasons for its failure and one main critical importance for its failure. The three critical reasons for its failure: 1) Napoleon greatly underestimated the fierceness and will of the Spanish fighting spirit. 2) Chiefly because of reason 1) above, the vastness of the Spanish frontier and the resources required to occupy & hold territory bled his army dry. 3) Napoleon did not expect the coalition arrayed against him to hold as strongly as it did. The main importance of its failure was that, due to Napoleon's overly ambitious tenancy to overstretch and string-out his resources, his other armies in other theaters of war were left under strengthened.
