Peyron attended the Royal Swedish Army Staff College from 1910 to 1912, was an aspirant in the General Staff from 1913 to 1915 and was appointed captain of the General Staff 1917. He returned the same year as "ryttmästare to the Life Regiment Dragoons. Peyron, who had left the Swedish Army in 1918, took part in the Finnish Civil War as a General Staff officer at headquarters. He participated there, among other things, to the plans of the Tampere and the Vyborg operations and was appointed Finnish lieutenant colonel. Back in the Swedish Army, he served as a military attaché in Rome from 1919 to 1921, became a major in the army in 1924, was a military attaché in Berlin from 1924 to 26 and chief of staff at the Cavalry Inspectorate ("Kavalleriinspektionen") from 1926 to 1932. He was promoted to major in the General Staff in 1926 and lieutenant colonel in 1928.