The Spanish Golden Age army had been forged from almost seven centuries of fighting with Muslim forces in what was called the Reconquista. This long term conflict resulted in the reunification of Spain under Christian rule and the end of the Moors in Spain.
From the site:
The military history of the XVI and XVII centuries has his importance with the "military revolution" from the middle age small armies to the huge Napoleonic army of the XVIII century. Technical progress of the portable firearms and tactical knowledge had reduced the importance of the heavy medieval cavalry, introduced the importance of the artillery in the battlefield and positioned the infantry as the main corps in the army.
The Spanish Tercios were the new infantry unit of the Kings of Spain which combined the firepower of the harquebus (later the musket) with the pikes. The Tercios would form the elite forces of the Spanish monarchy during 200 years. Contrary to others troops they fought only for one master, the King of Spain.