He played a pivotal role during World War II, serving in the Ski Battalion and conducting intelligence operations in Northern Norway after the German occupation of Narvik.

Synnergren's rapid ascent through the ranks continued, including graduation from the Royal Swedish Army Staff College and various military studies abroad.

Notably, he became the Chief of the Army Staff in 1963 and ultimately achieved the rank of Supreme Commander of the Swedish Armed Forces in 1967.

During his tenure as Supreme Commander, Synnergren oversaw significant defense policy changes, including the abandonment of neutrality and the development of the Saab 37 Viggen aircraft.

[4] The turning point of his life happened during a fall day in 1936 during the Berlin Olympics in which Synnergren participated as a member of the Swedish gymnastics squad.

The experience of Adolf Hitler, surrounded by the Third Reich throughout threatening paramilitary splendor changed Synnergren's choice of career.

In 1939, as World War II broke out, he finished first in his class at the Swedish Infantry Officer Candidate School at the Royal Military Academy.

[4] Synnergren was commissioned as an officer with the rank of second lieutenant in 1939 and served in the Ski Battalion (I 19 K) in Kiruna during World War II and guarded the Norwegian border when Germany occupied Narvik.

[7] Because of his fell experience and skills, he became, after a brief interlude as a cadet officer at Karlberg in 1944, the head of evacuation in Jokkmokk and was as such responsible for management of the Norwegian refugees.

[9] During Synnergren's time as the Supreme Commander, a series of long-term decisions were made that meant big changes for the defense.