From 1611 to 1721, Sweden was a European great power, becoming a dominant faction in the quest for control of the Baltic Sea and a formidable military power. During this period, known as "Stormaktstiden" (), the Swedish Empire held a territory more than twice the size of its modern borders and one of the most successful military forces at the time, proving itself on numerous occasions on battlefields such as Wallhof, Narva and Düna. The military of the Swedish empire is commonly (and wrongfully) recognized only as the Caroleans, which were in fact not in service until the late 17th century under Karl XI and his successor. The Swedish Empire and its modern military force was founded by Gustavus Adolphus, who inherited the throne in 1611 at age 17. He immediately reformed the common European military based on mercenaries to a professional national army. However, before completing his vision of conquering the Holy Roman Empire, the warrior king was killed in action in 1632. His daughter and successor did little to improve Sweden's military position and abdicated early, providing the Swedish Empire with a more warlike ruler. Karl X Gustav was only king for 5 years, but conquered large amounts of territory that still belong to Sweden today (including Blekinge, Bohuslän, Skåne and Halland). His son Karl XI would further strengthen the army by introducing the Caroleans, which were also used by Karl XII in the Great Northern War.