Final answer:
Gustavus Adolphus was the Swedish king and military innovator who led the Protestant forces during the Thirty Years' War.
Step-by-step explanation:
The Swedish king and military innovator who was one of the most successful leaders of the Protestant forces in the Thirty Years' War was Gustavus Adolphus. He led Sweden to numerous victories and implemented military tactics that revolutionized warfare during that time period. His innovative use of combined arms and mobile artillery allowed him to defeat larger and more powerful armies.