Hwasser joined the Swedish troops as a military doctor in the War of the Sixth Coalition (part of the Napoleonic Wars) in 1813-14. He served in Germany and Norway. After his return to Sweden, he pursued further studies in surgery at the Karolinska Institute in Stockholm. In 1817, at the age of 26, he was appointed Professor of medicine in Turku. While he was there, he gradually changed his view of medicine, beginning to believe that humans caused their own illness, while healing was a divine force.