World War II: A squad of 250 Norwegian volunteers defending Hegra Fortress surrendered to a superior Nazi force after a 25-day siege.

World War II or the Second World War was a global conflict between two coalitions: the Allies and the Axis powers. Nearly all of the world's countries participated, with many nations mobilising all resources in pursuit of total war. Tanks and aircraft played major roles, enabling the strategic bombing of cities and delivery of the first and only nuclear weapons ever used in war. World War II is the deadliest conflict in history, causing the death of 70 to 85 million people, more than half of whom were civilians. Millions died in genocides, including the Holocaust, and by massacres, starvation, and disease. After the Allied victory, Germany, Austria, Japan, and Korea were occupied, and German and Japanese leaders were tried for war crimes.

A volunteer military system or all-volunteer military system (AVMS) is a military service system that maintains the military only with applicants without conscription. A country may offer attractive pay and benefits through military recruitment to attract potential recruits. Many countries with volunteer militaries reserve the right to renew conscription in the event of an emergency. The Indian Army is the world's largest standing volunteer army.

Hegra Fortress is a small mountain fortress in the village of Hegra in Stjørdal Municipality in Trøndelag county, Norway. Originally known as Ingstadkleiven Fort, it was built between 1908–1910 as a border fort as a defence against the perceived threat of a Swedish invasion.

The Battle of Hegra Fortress was a 25-day engagement in the 1940 Norwegian campaign which saw a small force of Norwegian volunteers fighting numerically superior German forces from a fortified position. After initial fighting around the Meråker Line railway line, the Norwegians pulled back into Hegra Fortress and held off further German attacks before surrendering on 5 May as one of the last Norwegian units active in southern Norway.