On 1 August 1914, Germany declared war on Russia at the start of World War I. 
Groundstroem left his theological studies and joined the 27th Jäger Battalion (Finland) as a volunteer on 29 December 1915.


The Jäger Movement consisted of Finnish volunteers who had slipped away to Germany to train as soldiers and to fight during World War I.
Many of these volunteers were motivated by the goal of obtaining Finnish independence from the Russian Empire.

Groundstroem was made a lieutenant.

He fought in battles on the German Eastern Front on the Misa River, the Gulf of Riga and the Gauja.

Groundstroem married Elisabeth Rolfs, a German woman, in 1918.

Groundstroem was influenced by a militarism that he thought beneficial to young men, including "country boys" as well as "bookworms and spoilt, sloppy idlers".

In his view, military training, besides preparing a person for the future, built muscle and character.

In 1919 he wrote that military service can build an unshakable sense of duty in the individual.
The barracks life, where many conscripts live close together, removes pettiness, selfishness and vanity.

In another tract, however, he warns recruits of the dangers of barracks life.

After the October Revolution in Russia, the Finnish socialists followed Vladimir Lenin's urging and on 9 November 1917 set up a Revolutionary Central Council of Workers in Helsinki.
In December the center-right coalition that controlled parliament declared independence.

In January the socialists seized control of Helsinki and announced the Finnish Socialist Workers' Republic.

The "White" parliamentary government fled Helsinki.

They named Carl Gustaf Emil Mannerheim commander in chief, and he quickly took control of central and northern Finland.

Mannerheim, from the Swedish-speaking elite, formed his high command from 84 volunteers from the Swedish army, and his officer corps from 1,130 Jäger volunteers.

Groundstroem returned to Vaasa, the temporary capital of Finland in the White region, and on 25 February 1918 was promoted to lieutenant.

In March 1918 he was appointed a battalion commander in the White forces, taking part in battles in Tampere, Lahti, Lyykylä, Mannikkala and Tali, where he was slightly wounded.

The civil war ended on 15 May, when the Whites took over Fort Ino, a Russian coastal artillery base on the Karelian Isthmus, from the Russian troops.
White Finland and General Mannerheim celebrated the victory with a large military parade in Helsinki on 16 May 1918.

Groundstroem served in the General Staff organization from 3 June 1918 until he resigned from the army on 26 June 1919.
On 1 August 1914, Germany declared war on Russia at the start of World War I. 
Groundstroem left his theological studies and joined the 27th Jäger Battalion (Finland) as a volunteer on 29 December 1915.

The Jäger Movement consisted of Finnish volunteers who had slipped away to Germany to train as soldiers and to fight during World War I.
Many of these volunteers were motivated by the goal of obtaining Finnish independence from the Russian Empire.
Groundstroem was made a lieutenant.
He fought in battles on the German Eastern Front on the Misa River, the Gulf of Riga and the Gauja.
Groundstroem married Elisabeth Rolfs, a German woman, in 1918.
Groundstroem was influenced by a militarism that he thought beneficial to young men, including "country boys" as well as "bookworms and spoilt, sloppy idlers".
In his view, military training, besides preparing a person for the future, built muscle and character.
In 1919 he wrote that military service can build an unshakable sense of duty in the individual. The barracks life, where many conscripts live close together, removes pettiness, selfishness and vanity.
In another tract, however, he warns recruits of the dangers of barracks life.
After the October Revolution in Russia, the Finnish socialists followed Vladimir Lenin's urging and on 9 November 1917 set up a Revolutionary Central Council of Workers in Helsinki. In December the center-right coalition that controlled parliament declared independence.
In January the socialists seized control of Helsinki and announced the Finnish Socialist Workers' Republic.
The "White" parliamentary government fled Helsinki.
They named Carl Gustaf Emil Mannerheim commander in chief, and he quickly took control of central and northern Finland. 
Mannerheim, from the Swedish-speaking elite, formed his high command from 84 volunteers from the Swedish army, and his officer corps from 1,130 Jäger volunteers.
Groundstroem returned to Vaasa, the temporary capital of Finland in the White region, and on 25 February 1918 was promoted to lieutenant.
In March 1918 he was appointed a battalion commander in the White forces, taking part in battles in Tampere, Lahti, Lyykylä, Mannikkala and Tali, where he was slightly wounded.
The civil war ended on 15 May, when the Whites took over Fort Ino, a Russian coastal artillery base on the Karelian Isthmus, from the Russian troops. White Finland and General Mannerheim celebrated the victory with a large military parade in Helsinki on 16 May 1918.
Groundstroem served in the General Staff organization from 3 June 1918 until he resigned from the army on 26 June 1919.
Kalervo Kurkiala