After beginnings as a teacher in Türi, he served with the Imperial Russian Army in World War I, and was advanced to Staff Captain.

During the disintegration of the Russian Republic in 1917, he returned home to establish a branch of the Estonian Defence League.

His final contributions were as the last Chief of Staff of the Estonian Army, in which capacity he served from 1939 to 1940, reaching the rank of major general.

He studied in the 4th Petrograd Praporshchik school, receiving the corresponding rank in the Imperial Russian Army, and was then called up to fight on the Eastern Front against the German Empire.

[2] In December 1917, shortly after the October Revolution, he joined the Estonian national reserve battalion of Tartu, though this was disbanded in March of the following year.

[4][2] He fought against both the Red Army and the Baltische Landeswehr, and suffered a concussion in June 1919 near Stalbe in Latvia.

[2] For his service in the War of Independence Jaakson was awarded Estonia's Cross of Liberty and Latvia's Order of Lāčplēsis, as well as a farm and 50,000 marks.

[5][7] Jaakson participated in public life belonging to several organisations, including being an honorary member of Estonian Naturalists' Society.