It only became of importance when the Norwegian artillery major Hans Reidar Holtermann started organising troops to resist the German invasion forces which had been landed at Trondheim.

From 12 April work was carried out to reactivate the fortress's artillery, which was found to have plentiful ammunition, but no direction systems or charts for indirect fire.

The same day troops of Holtermann's unit were positioned around Hegra railway station and Mælen bridge, and the first German attempt at making the fortress surrender was carried out.

Through a telephone conversation, the commander of the force at Hegra was told to act as he saw best and, if possible, to hinder the Germans in gaining control of the Meråkerbanen railway line to Sweden.

In response to these orders, 20 soldiers were sent to the village of Flornes to set up field fortifications and block the road and railway to Meråker Municipality.

[24][25] Later that afternoon, the garrison's sole female member joined when nurse Anne Margrethe Bang from Trondheim arrived at the fortress bearing a load of medical supplies.

Having been partly caught by surprise, the Norwegian forces at the Hegra road bridge and the railway station made a fighting retreat to the fortress over a two- to three-hour period.

Early on in the fighting, the Norwegians demolished the Hegra road bridge,[28] forcing the German infantry to cross the precarious ice of the frozen Stjørdal River under fire.

[9][30] At the end of the first day of serious fighting, the Germans pushed on along the Meråkerbanen railway line and broke through the blocking position at Flornes.

As night fell, German troops had occupied the areas around the villages of Hegra, Avelsgaard, Flornes, Ingstad and Sona.

[30] The day after the German capture of the area surrounding the fortress, Luftwaffe aircraft repeatedly attacked with bombs and machine gun fire.

At 09:00, a large force of German infantry attacked from the north-east, supported by machine gun positions situated a mere 150 m (160 yd) north of the fortress.

At this point, the attacking force was subjected to heavy fire at close range from artillery, machine guns and riflemen, and thrown back.

During the previous days' fighting many German wounded had been brought to Hegra village and the doctors feared that there had been numerous casualties on the Norwegian side as well.

While Aarrestad led the expedition, Berdal was held hostage by the Germans to ensure that the Norwegians returned from the fortress after finishing their mission.

The pressing need to remove the Norwegian force ended in large part when the important town of Steinkjer fell to the Germans on 21 April and the Allied advance from the north was checked.

The southern arm of the Allied counter-attack had never swung north from Åndalsnes and had instead been directed to the Gudbrandsdal in order to support the Norwegian forces fighting there.

The fortress was under constant artillery fire and held out chiefly in order to be in a position to support the Allied offensive expected from the north.

[38][39] On 25 April, the Germans employed a new weapon against the fortress when a seaplane dropped a 1,800 kg (4,000 lb) bomb, destroying the houses outside the walls, with shrapnel ending up in Hegra village several kilometres away.

Recognising this, the Germans had hired some 2,000 Norwegian collaborationist labourers to work full-time at expanding and improving the air strip.

Bombarding Værnes would both have disrupted this work and impaired the bombing raids being flown against Norwegian forces fighting further to the north.

[51] News came in over the radio on 2 May of the Allied retreat from the Åndalsnes area, that the Germans had seized control of the Dovre Line from Dombås to Støren, and of the surrender of the Norwegian 4th Brigade in Western Norway.

The decision to surrender Trøndelag had been influenced by Neville Chamberlain's radio message that day announcing the end of the Allied campaign in Southern Norway.

[53] On 4 May, destruction of radios, machine guns, carbines and other small arms was initiated and ski patrols were sent out carrying loads of important documents.

[54] In the early hours of 5 May, the situation was such that there was very little food left and water for only a few days, all other Norwegian and Allied forces in all of southern Norway had been withdrawn or had surrendered and Hegra Fortress was the last remaining pocket of resistance south of Nordland.

By 05:25, a white flag was raised over the fortress, and at 06:30 a force of 60 German soldiers and three officers[55]—led by one Hauptmann Giebel[6]—arrived to accept the surrender of the garrison.

The road was intended to help the German logistic system that had been severely hampered by the numerous bridges that had been blown by the Norwegian Army during the preceding month.

Due in part to the poor physical condition of the prisoners after the harsh siege they had just experienced, the road was never completed despite threats of punishment made by the German camp commander.

[57] At the end of May the German High Command in Berlin announced that Adolf Hitler had personally ordered the release of Norwegian prisoners of war as an act of recognition of the defence they had put up under difficult conditions.

[58] In the first years after World War II, Norwegian estimates of the number of German casualties were exaggerated, some spoke of up to 1,100 dead or wounded.