Final answer:
Gustavus Adolphus was the Swedish king who introduced innovative infantry and cavalry tactics during the Thirty Years' War on behalf of the Protestants.
Step-by-step explanation:
The Swedish king who was a military innovator and developed more flexible infantry and cavalry tactics during the Thirty Years' War on behalf of the Protestants was Gustavus Adolphus. He implemented the use of longer spears for infantry, which held the enemy in place while the cavalry charged, a tactic that proved effective against various adversaries. Gustavus Adolphus also emphasized the drilling and training of his infantry, resulting in a formidable and successful army.