The war fought in spain was significant because it:

Indicated that Napoleon could be defeated on the battlefield.

Explanation:

The French armies entered Spanish territory under the supposed reinforcing of presence in Portugal, yet they diverted to Madrid and engaged in a four-year war that they lost.

The armies following Napoleon's orders with opposing Joachim Muram were demoralized with the overall loss and the reputation of the French was contested.

Napoleon had sought cooperation from the Spanish to invade Portugal yet it was denied.

The battle was made with support from the British and the Portuguese who were a major faction against Napoleon's campaign.

After invading Spain, the consecutive defeat at Waterloo came to end any possibility for Napoleon to recover after his return.

With the formal abdication four days later from Waterloo, Paris got a new government and a return to old regimes.

In Spain is called "Guerra de Independencia" or War of Independence, and it also created geopolitical conditions in the Americas for colonies to seek independence taking advantage of Spain's situation correspondently.

c) troops march forward in close formation against newer, more accurate weapons