While in Germany, the Finnish volunteers formed the 27th Royal Prussian Jäger Battalion, fighting for the Imperial German Army on the Eastern Front of World War I.

[2] In 1933, Lagus was made the commander of a supply battalion, later regiment, holding the posting until the start of the Finno-Soviet Winter War in late 1939.

[2][3] In the Continuation War, where Finland attacked the Soviet Union alongside Germany, Lagus initially led the 1st Jäger Brigade in the Finnish invasion of Ladoga Karelia.

During this initial offensive phase, Lagus was briefly given command of the 5th Division, but had to relinquish the posting to Ilmari Karhu who was preferred by the Finnish commander-in-chief Marshal Carl Gustav Emil Mannerheim.

[2][3][8] Following his retirement, Lagus became the chief executive officer of Lohjan Sato Oy, a Finnish limited company which built social housing in the Lohja region.

His first marriage, with Olga Johanna (Jane) Ramsay ran from 1921 to 1927, after which he married Kenny Christine Emilia Gadd in 1935.