One of the most impressive military innovations of Gustav II was...

One of the most impressive military innovations of Gustav II was the resurgence of cavalry. During the Early Modern Period, the development of pike tactics and especially the Spanish tercio (which combined pikes with firearms) severely threatened the previous supremacy of cavalry (disciplined and well-trained blocks of pikemen were effectively impervious to cavalry attacks). Cavalry was forced to wait until pike squares were broken before charging, so they developed a tactic of "rolling fire" where multiple ranks of pistol-armed horsemen would advance on the enemy, fire, and retreat to reload in sequence. In theory, this would weaken the pikemen sufficiently for a melee attack.

Gustav II abandoned this tactic and instead had his cavalry charge the enemy, fire point-blank, and continue on into melee range with swords. Combined with the firepower of musketeers and artillery, the cavalry were able to break the pike formations and close effectively.