The campaign of 1809 proved the wisdom of this policy, as they effectually prevented Prussia from joining Austria in rekindling the flames of war.

And again in 1813, these works might have produced a decided influence on the campaign, had not the political perfidy of Austria, and the treason of the French generals, prevented Napoleon from profiting by the advantages of his position.
The influence of the fortifications of Spain upon the Peninsular campaigns has often been alluded to by historians.

Those works which had been given up to Napoleon previous to the opening of hostilities, contributed very much to the success of his arms; while those which had been retained by Spain and her allies contributed in an equal degree to fetter and embarrass his operations.

Some of these, like Saragossa, Tarragona, Gerona, Tortosa, &c.

&c., with their broken walls and defective armaments, kept the enemy in check for months; and, by compelling the French to resort to the tedious operations of sieges, did much to weaken the French power in the Peninsula.
The influence of the fortifications of the French frontiers in furnishing a secure basis for the successful operations of Napoleon into the enemy's territory, has already been noticed.