There he attended the École spéciale militaire de Saint-Cyr in France for three years from 1924 to 1926,[2][12] gaining the French certificate of Brevet d'état-major.

[11] As the German Gruppe 2 broke through the Norwegian coastal forts defending Trondheim major Holtermann was ordered to take part in the mobilisation of Artillery Regiment no.

As the Germans advanced on this important army camp it became impossible to complete the mobilisation there and Holtermann sought out a new and more secure location to organize the troops.

At Hegra Holtermann organised an improvised force of 250 soldiers and one female volunteer auxiliary with the intention to resist the invaders until effective support could arrive from elsewhere.

As the tide of war developed unfavorably for the Norwegian forces in the south of Norway major Holtermann made it his main mission to hold the fort until the expected allied thrust from the north reached his area of operation.

However, since the artillery pieces at the fortress were intended to fire eastwards towards Sweden, not westwards towards Trondheim, the guns at Holtermann's disposal could not reach the German held air strips.

For twenty-five days Holtermann and his small force held out under continuous artillery and Luftwaffe bombardment in what became known as the Battle of Hegra Fortress.

In the end the fortress' significance consisted mostly of serving as a morale booster for the Norwegian population in a time of mostly bad news.

Following his release in June 1940 Holtermann did not resign and accept the German occupation of his homeland but quickly became active in the fledgling Norwegian resistance movement.

In 1944 Holtermann was transferred to lead District Command Trøndelag and returned to Sweden where he took part in Norwegian preparations for the coming liberation of Norway.

[11] In the citation to the award of the War Cross with sword to Hans Reidar Holtermann the Norwegian Army High Command wrote: