La Guardia Civil (English: The Civil Guard) is a decision available to 
Spain. It marks the formation of the Spanish Civil Guard.
It requires that Rebels do not control a single province. On top of that either should the average militancy be at least 3.0 or the average consciousness should be at least 4.0. There can be active rebel troops in the country, they just need to not have occupied any provinces. Similarly, even if you have removed all rebel stacks, you cannot take it if they have occupied a province.
It gives 1% extra tax efficiency, without any drawbacks.
The In-game text states:
"La Guardia Civil', Spanish for 'The Civil Guard', is the Spanish gendarmerie corps, established during the reign of Queen Isabella II to suppress lingering Carlist sentiments in the countryside after the Liberal victory in the Carlist wars. The corps is a blend between a police and a military force, operating on a federal level, and has remained in existence through to modern time."