Final answer:
Gustavus Adolphus was the king of Sweden from 1611-1632 who implemented crucial battlefield innovations and played a key role in the Thirty Years' War, defending Northern Protestant states and eventually dying at the Battle of Lützen.
Step-by-step explanation:
The leading Protestant ruler you are asking about is Gustavus Adolphus. He became the king of Sweden in 1611 and reigned until his death in 1632. Gustavus Adolphus is notably recognized for his significant contributions to military tactics during his time. He strategically intervened in the Thirty Years' War, a major conflict that spanned from 1618 to 1648, which was primarily between Catholic and Protestant states within the Holy Roman Empire. His interventions were aimed at protecting the northern Protestant states, and he was successful in securing notable victories for the Protestant cause before dying in the Battle of Lützen.